I don't know if there is some sort of flagging system to detect abuse, but I was able to buy a $25 Amazon GC (I needed to order then and there, so I didn't have time to run to the grocery store for the category bonus), but realized that I miscalculated and need more than $25. I went to buy another $25 Amazon GC, but selected another credit card. The app was processing forever until I closed it. The purchase never went through and I wasn't charged for it. I decided to redeem other rewards currencies for another Amazon GC instead.

EDIT: I spoke too soon. Tried to buy an Old Navy GC (paying with a Visa card on file) and received the "Transaction Declined - We're sorry -- your transaction cannot be completed. Please email [email protected] for assistance" message. I tried again with my Amex, which yielded the perpetual Processing Payment screen.

I think it's some sort of bug with the app. The app has turned off Touch ID for no reason. I re-enabled it, logged off, logged back in, and tried to buy another card where it mysteriously disabled it again. I've tried leaving Touch ID off and logged out, and logged back in with my password. No change, still can't use the app.

I've sent an email to the MPX Agent with the relevant info.