[mytailorisrich]

Both "s'inscrire à" and "s'inscrire dans" exist. The trick is to pick the right form. Both may be correct in some cases, but not in others.

Sometimes the meaning changes as well. For example "s'inscrire à l'école" and "s'inscrire dans une école". The latter means "to register with a school" but the former is more "to register to start school" in the sense that it is 'school' in general.

As for "crédits", indeed the meaning is basically the same as in English and the original sentence is correct in the language. But it's not something anyone would say. The terms used would most likely be either "bon" (voucher) or "forfait" (inclusive special rate), or perhaps "chèque" (cheque) instead of "bon" if it was given by your employer as a benefit.