You did check in but without checking out you do not pay the right amount. You probably paid too much. It may be corrected by the buscompany or by https://www.uitcheckgemist.nl 

It is a small amount probably, so maybe not worth the trouble. I presume you traveled with a balance.

This is a NS site.

For buses you should always ask for a refund via uitcheckgemist.nl for missed check outs.

Filling in the contact form for that will usually result in being referred to that site anyway. (The contact form of the bus operators is only to be used for corrections when the wrong fare is charged)