Hey rubywaitweg. Welcome to the NS Community. 

We can't look into your invoice here so I can't be sure, but if you've recently claimed something/gotten something refunded with our Customer Service I expect this is the reason for the refund. This is not added to your invoice, but subtracted from it.

Did you recently switch to NS Flex?

If I remember it correctly, on my invoice this was the description of the reimbursement of the credit (that is longer needed with NS Flex) that I had on my OV-chipkaart. As @Daniël NS pointed out already it is not a fee, but a reimbursement.