It is a good idea to upgrade your World of Warcraft account to Mists of Pandaria and not do it in the last days when Blizzard’s servers will get flooded by user requests that are trying to upgrade their game.
There is an issue that may be fixed sooner or later, if you enter your credit card with empty spaces (eg. 1254 6598 7896 1236), your credit card will get rejected.
This doesn’t happen if you pay for your monthly subscription or the remote smartphone application, if you just leave blank spaces for your credit card number, it will still work.
Other reasons for your credit card to be rejected. If you don’t have enough money on your credit card obviously will not work and your payment will get rejected.
Blizzard’s servers not working as they should will also reject your payment. If your order was processed, you should receive in like 5 or 10 minutes or sometimes instant, an e-mail confirmation with your order.
If your payment still gets rejected, call your bank support and ask if the transaction took place, if you have enough money into your account etc.
The standard upgrade for Mists of Pandaria doesn’t contain any in-game goodies, the deluxe version can be upgraded from the standard version for just 15 euro.
The deluxe version contains an in-game mount, some sort of ram with wings with some nice effects overall, a banner for Diablo 3 and some avatars for Starcraft 2.
More help from Blizzard you can get by submitting a support ticket from battle.net or calling them (can be done even by using Skype and talking in English).