> The fee would be paid at the point of converting your crypto to the 'Credits'(which users would happily pay, they already do when converting to Fiat)

These fees are going to have to be higher than normal conversion fees, if you want to fund the credits operation - someone is going to have to pay for -

Alterations to the PoS software to accept credits Server infrastructure which can authorise the transactions Helpdesk staff to help out the shop staff when they get confused/stuck/whatever