facturatie

Turn Mollie payments into invoices automatically

2026-09-30 · 7 min leestijd

Your own website takes payments through Mollie. The money arrives, but an invoice appears nowhere: your site does not create one, and an off-the-shelf plugin does not fit a site that you or an agency built from scratch. Yet the payment itself already holds much of what an invoice needs: the amount, the payment date, the customer and often the VAT details too. slimzaak's Mollie Facturatie extension turns that into a proper invoice. Below you can read how it works, what your site should put in the payment, and when this is not the right route.

Why a payment in Mollie is not yet an invoice

Mollie is a payment provider. A payment there is proof that money changed hands, not an accounting document. An invoice needs more: a continuous invoice number, your company details, your customer's details and a VAT breakdown. If you want those requirements in one place, read our guide to making an invoice.

With a webshop platform such as Shopify or WooCommerce, the integration fills that gap: the order supplies the data and the invoice follows from it. A custom website has no such layer. There is only a payment, and it often holds more than you think: the amount, the time, a description, the billing address and, if your site sends it along, the product lines with their VAT rate.

Mollie Facturatie builds on that payment. Your site stays as it is; slimzaak reads what was paid from your Mollie account afterwards.

How it works, from connection to invoice

Mollie Facturatie is an app extension in slimzaak (its page is titled Mollie Invoicing in the English interface). You connect your own Mollie account through OAuth: you log in to Mollie and give slimzaak read access to your payments, refunds and Mollie invoices. slimzaak cannot pay or change anything in your Mollie account. You withdraw the access through Mollie, or with Disconnect in slimzaak.

After that, slimzaak fetches new payments every half hour; with Fetch now you do it straight away. Only paid payments count, and only those from live mode: slimzaak never sees test payments. The slimzaak interface is available in Dutch and English, with Dutch as the default; the labels below are the English ones.

What your website puts in the payment

Your website does not have to pass anything to slimzaak, but the payment it creates at Mollie is the source of the invoice. The more you put in, the less you have to complete later. These are the fields slimzaak reads.

VAT: which rate goes on the invoice?

VAT is the part of an invoice you do not want to gamble on, and slimzaak does not gamble. The rate of a payment is decided in this order:

1. If your site sends lines with a vatRate, the rate of each line applies.
2. If the payment arrives as a single amount without lines, slimzaak uses a rate you chose earlier for the same description, and otherwise your fixed choice: everything 0%, 9% or 21% (the Dutch rates), if all your sales carry the same rate.
3. If you pick a rate yourself when invoicing, it applies to that payment and we remember it for that description.

If none of these exists, for instance with lines that carry no rate, the invoice becomes a draft that says what is missing: the VAT rate or the customer's country. You fill it in and issue the invoice. So an issued invoice never carries a guessed rate.

The invoice is built from gross amounts: what your customer paid is what the invoice says, VAT included. For a customer outside the Netherlands, slimzaak looks at the country and the type of customer. A business customer from another EU country with a VAT number gets VAT reverse-charged (0%), a customer outside the EU gets 0%. If OSS is on in your settings, slimzaak converts the rate you set for a payment without lines into the rate of the consumer's country in the same group (standard or reduced). How that scheme works is explained in the OSS scheme for webshops.

Review first or automatic: you choose

As soon as you connect Mollie, slimzaak creates no invoices or expenses yet. Everything it finds goes on a list, with what would happen for each payment, for instance Becomes invoice + order. You choose, per payment or for a group at once: Invoice, Ignore or Link. That is a deliberate choice, because an issued invoice can no longer be deleted.

The list also helps with what you already have:

• Already made an invoice yourself? If an invoice of yours carries the payment id (tr_…), we link it to the payment instead of making a second one. If there is an invoice for the same amount within a week of the payment, we suggest it as a link.
• Payments that are not sales? Choose Ignore, also from now on, and similar payments (same description, without an order number) are skipped.
• Unsure about a payment? Open the details: who paid, which lines, which rate and which totals. That overview is calculated by the same code that makes the invoice, so you see beforehand what you will get.

When you are done reviewing, switch the mode to Automatic. New payments then become an invoice straight away and new Mollie invoices an expense straight away. What was already on the list keeps waiting for your choice.

What else happens: customer, refunds and Mollie fees

An invoice is not the only thing that follows from a payment:

• Customer and order. If the payment carries an e-mail address or a Mollie customer, the payer becomes a customer in slimzaak. The payment appears as a completed, paid order under your orders and revenue.
• Refunds. If you refund a payment in full or in part in Mollie, slimzaak makes a credit note attached to the invoice. Read more about credit notes in cash book and credit notes. slimzaak sees refunds on payments from the last 60 days. If the invoice is still a draft, the credit note follows only after you have issued it, at the next fetch.
• Mollie fees. The invoices Mollie sends you, that is the transaction fees, land as an expense in your administration, with Mollie B.V. as the supplier and the PDF attached when Mollie provides it, from the month of your start date. There too you choose yourself what gets booked while on Review first.
• Numbering. An invoice gets the next number in your company's continuous sequence, with a PDF attached.

When this is not the right route

This extension does not fit every situation, and we would rather say so up front:

• Your site runs on Shopify, WooCommerce or another connected webshop. Then the webshop integration invoices the order, and slimzaak recognises the Mollie payment as belonging to it (through order_id in the metadata, or Order #1043 in the description), so nothing is invoiced twice. Only a manual store invoices a payment itself. If a profile also belongs to a webshop, a payment without an order waits up to three days for that order.
• Moneybird makes your invoices. If Moneybird is set up as the party that invoices, Mollie Facturatie does not make slimzaak invoices: those numbers would land in books you handed over to Moneybird. See slimzaak and Moneybird.
• Your customer must get the invoice in another language. The invoices are currently drawn up in Dutch.
• You have a handful of payments a month. Then making an invoice yourself may be enough, for instance with our guide to making an invoice.
• You do not use Mollie. This extension only reads Mollie.
• Chargebacks. The extension does not process those yet; a refund you make yourself, it does.

If you only sell through a connected webshop or marketplace, you do not need this extension. If you also take payments on your own website, it fills exactly that gap.