We pay ours online. I book either with a rep or online and pay as I want. I had this issue with my card getting declined two years ago and it was a glitch in Carnival's system. I have had where the postal code is entered wrong and when corrected it goes through. I also don't pay full cruise at one time. Just the deposit and then when I have funds charge it and pay back as soon as I know the exchanged amount. I also prepay everything I can.
I prepay tips, excursions, drink package, transfers and pre-load cruise cash. We also only use a cash account on the ship.