Can I import my old invoices into Sum?

Only the unpaid ones. In Settings, then Opening Balance, you paste each open invoice as one line: customer, invoice number, issue date, due date and the amount still owed. Paid invoices stay in your spreadsheet and your PDFs, which remain your archive.

Can I keep my own invoice numbers?

Your open invoices keep their old numbers in the opening balance. New invoices follow Sum's own series, starting at INV-0001. Sum gives the number when you send, so a deleted draft leaves no gap. You can't set a different prefix or starting number.

Does Sum read a CSV that Excel saved with semicolons?

Yes. Sum detects whether your file uses commas, semicolons or tabs. Columns named as in the template map themselves. A column with another name, in Dutch for example, you map by hand from a dropdown before the preview.

Invoice spreadsheet to invoicing software: move to Sum

Q: Does Sum cost extra?

No. Sum is in every Lyto plan, together with the other six apps. On Seed, for one to five people, a seat costs €10 per month, billed annually, or €13 billed monthly. The 14-day trial needs no card, so you can import your own customer list before you pay.

An invoice spreadsheet works while you send a few invoices a month and remember who paid. It stops working once you number invoices by hand, copy VAT formulas and chase payment from memory. Lyto Sum takes over in two imports. Your customers come in as a CSV file. The invoices still unpaid go into the opening balance. From then on, Sum numbers every new invoice itself.

Many freelancers and small teams send their first invoices from a spreadsheet. One tab holds the customers, another holds a row per invoice, and a template turns a row into a PDF. This guide covers when that setup starts costing you money, and how to move it into Lyto Sum.

When does an invoice spreadsheet stop working?

Look at the end of the month. A spreadsheet is fine as long as none of these apply:

You type invoice numbers by hand. A skipped or doubled number is a gap your tax office can ask about.
VAT is a formula you copy. If one row has the wrong rate, or a formula was pasted over, the total is off.
You chase payment from memory. Nothing tells you an invoice is 14 days late, so the reminder goes out when you remember.
Paid and unpaid live in one column. You tick a cell when the money arrives, and you check your bank to find out which cell to tick.
You copy hours into the invoice. If you track time in a separate sheet, every copy is a chance to drop a line.

If none of these happen, keep the spreadsheet. Switch when they start.

What does invoicing software add?

Sum is the invoicing app in Lyto. You pick a customer, add lines, and Sum works out the VAT per line. Before an invoice goes out, Sum checks that the legally required details are on it. If one is missing, Sum does not send it and says what is missing.

Sum numbers each invoice when you send it, so a draft you delete costs no number. It emails the invoice as a PDF. Sum comes with a reminder schedule: a reminder, a second reminder and a final notice, at 3, 14 and 30 days past the due date. You turn on automatic sending, or send each reminder yourself.

An invoice is settled when its payment shows up in your bank transactions and you confirm the match. If you connect Mollie, each invoice carries a payment link. The aged receivables report shows who still owes you, and for how long.

Hours you log in Tally go to Sum as a draft invoice in one click. If those hours live in a spreadsheet too, the guide to moving an Excel timesheet into Tally covers that import.

What moves, and what stays behind

Sum takes two things from your spreadsheet. Your customers come in as a CSV file. The invoices that are still unpaid come in as open items in the opening balance, so Sum can chase them.

Invoices that are already paid stay where they are. Sum has no import for invoice history. Keep the spreadsheet and the PDFs you sent as your archive.

Step 1: get the customer list in shape

The importer reads one row per customer. The template below has nine columns, with two example rows. Only display_name is required.

Column	Required	What goes in it
display_name	Yes	The name you see in Sum, once per customer
legal_name	No	The registered name, if it differs
kind	No	company or individual; anything else imports as company
country_code	No	Two capital letters, such as NL or DE
vat_number	No	The customer’s VAT number
primary_email	No	Where invoices and reminders go
primary_phone	No	A phone number
website	No	The customer’s website
payment_terms_days	No	Days until payment is due; empty means 30

Columns named exactly like this map themselves. If your sheet uses other names, you map them by hand in the next step. Sum reads CSV files with commas, semicolons or tabs, so Excel’s semicolon export works too. One file holds at most 5 MB and 1,000 customers.

Write the country code in capitals. A row with nl in place of NL fails.

Step 2: import your customers

Open Sales, then Customers, in Sum and click Import CSV. You need the owner, admin or bookkeeper role. The import has three steps:

Upload and map. Upload the file and check that each column points at the right field. A column you have no use for gets “(skip this column)”.
Preview. Sum checks every row against the customers you already have. It looks for the same VAT number first, then the same email, then a very similar name. Each row shows as new, matched, a duplicate within the file, or an error. A matched customer keeps its details and takes the payment terms from your file.
Import. Sum saves the customers. Rows that fail are listed on screen, each with a reason. Fix those rows in your sheet and import them again.

The same import exists for suppliers, under Costs, then Suppliers.

Step 3: bring in the unpaid invoices

Open Settings, then Opening Balance. First enter your switch date and the balances you start with, and post them. Your accountant or last year’s balance sheet has those figures. The box for open invoices stays locked until the opening balance is posted.

Then paste one line per unpaid invoice, in this order: customer name, invoice number, issue date, due date and the amount still owed. A line looks like this: Example Client,2026-031,2026-09-15,2026-10-15,1210.00.

Dates are written as YYYY-MM-DD. An empty due date takes the issue date.
The amount is the total still owed, VAT included, with a dot for the decimals.
The invoice number stays as it was. A number that looks like Sum’s own series, such as INV-0001, is refused.
The customer is matched by a very similar name, or created.

One paste holds at most 500 invoices. These open items count as money owed to you, so they appear in the aged receivables report. Sum sends reminders for them to every customer with an email address. Import your customers in step 2 first, so each invoice finds its customer by name. You booked their revenue and VAT when you sent them, so Sum books neither again.

Step 4: switch over

From the switch date, raise every new invoice in Sum. Its numbers start at INV-0001, and you can’t change the prefix or the starting number. Your old invoices keep their own numbers, so you now have two series: the old one, which ends, and Sum’s, which runs on.

What Sum does not do

Sum does not import paid invoices or the lines of old invoices. It has no recurring invoices yet, no quotes and no e-invoicing over Peppol. Sum works in euros only. There is no button that marks an invoice paid. Payment comes from a confirmed bank match, a Mollie payment or a credit note.

What it costs

Sum is in every Lyto plan, with the other six apps. On Seed, for one to five people, a seat costs €10 per month, billed annually, or €13 billed monthly. Larger teams and the other plan are on the pricing page. The trial runs 14 days and needs no card, so you can import your own customers before you decide.