are you saying your Serve card run as credit went through as payment for MO? I know it's possible, I've been to stores that accepted CC for buying MO, but if that's the case for you, you'd be better off using other CCs with greater rewards in miles/points/CB since you don't earn anything from using Serve unless there's a specific am3x sync offer on that particular Serve card.

Otherwise, IME buying MOs when registers are hardcoded to accept cash and/or debit cards only, CCs are rejected once swiped.