Installing the integration starts with signing up for a trial period via the integrations page of winkelboekhouding.nl. After signing up, you will receive an e-mail with an installation URL and login details to start the installation. (You can also always find all winkelboekhouding.nl guides on winkelboekhouding.nl.)
You can try the integration free of charge for the first 30 days. After that, the integration does not automatically become paid: you will first receive an e-mail asking you to choose a pricing plan based on the number of invoices you transfer. View the pricing of winkelboekhouding.nl here.