Be careful with such deals. Credit cards are charged on monthly subscription. Even though OP's service will be a one time payment, there is no guarantee he wouldn't generate a huge bill for the next billing cycle which may result in card getting charged the next month.Also the card details will be saved on the profile which I suppose can also be easy to unauthorised access.
I wouldn't recommend anyone doing this just for a couple of bucks.
Nice catch there. When I read the offer, I thought there is no way OP can cheat. But you are right, this could also be a scenario. And even if OP doesn't have any intention, there are websites that do such debits without even asking. It has happened to me with WIX account. As for OP has intention to cheat or not, I'll say no. Seems like she is a model and will be earning, not spending!!