You can find your receipt in MijnNS.

As MoDe mentioned, you can find a detailed invoice in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments) to see what you are being charged for. Perhaps some correction fees are applied, or travel costs incurred that your season ticket doesn't cover.

If after looking at the invoice you still feel that you are being charged too much, or if you can't find the detailed invoice, then you should contact customer service directly. They can tell you what the higher amount is for, and correct an incorrect invoice.

Usually, contacting customer service this is most easily done by phone, as they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.