In EU, this depends mainly on the question of exchange/interchangeability: If you sell them as vouchers and do not allow redeeming/payout in the original cash, its not a problem.

The key legal issue is interchangeability. Single-merchant vouchers are generally acceptable. If a voucher can be used across multiple merchants, it's often treated as e-money in the EU. Not being able to use funds across multiple merchants would significantly reduce the value for customers, as they would no longer be able to share payment processing fees across merchants.