I've had this same problem recently - it seems to be caused by a credit card security function banks have implemented recently called "Secure 3D" on credit cards, where merchants like the Mogstation can use it to check to see if the transaction is valid - what is meant to happen is it's meant to connect to your bank's website that will ask you to input a one time code that will be sent to your phone or email address that is then put in on the frame that appears there. Once it's done it verifies and the transaction is allowed. I've been having nothing but trouble trying to get it to work on the latest version of Firefox, but using my mother's computer with Chrome it worked fine.

Unfortunately there is no way to opt out of this function if your bank/card vendor has activated it, and as stated tends to glitch a lot of the time. All you can do is try an alternative browser and see if that helps.