If there is an unrecognized charge on your card that shows it is from Stamp, please try to think about your recent tax free shopping history. The charge is definitely related to your tax free purchases.
If the charge is €3,99 or €9,99 or €19,99 or €24,99 or €39,99, it should be our service commission. Please read here for more information on our commissions: https://help.stampit.co/en/articles/3322350-how-much-does-stamp-charge
If the charge is not one of the above, it should be the VAT charge of a rejected tax free invoice, and is usually followed by another small amount charge, which is the fee for managing the account.
For more information on how Stamp works: