Hi Solomiia,

The monthly invoice date never changes. So if you change your subscription you'll get an intermediate invoice for the remaining days until the next monthly invoice date. On this next invoice date you'll get an invoice for a new month.

For example: you have Dal Voordeel and your monthly invoice date is the 18th. You change your subcription on March 8th to Dal Vrij. After the change you'll receive an intermediate invoice on which you'll have to pay Dal Vrij for 10 days (10/29 * 119,95 = 41,36) and the Dal Voordeel will be credited for these days (10/29 * 5,60 = 1,93). Total 41,36 - 1,93 = € 39,43.
On March 18th you'll receive another invoice for a full month Dal Vrij = € 119,95.

You can download the invoices from Mijn NS under ‘Betaaloverzicht / My payments'. All costs are specified on the invoice, including the dates you've been charged for.