So, I was trying to purchase the Warlords of Draenor Digital Deluxe Items that went on sale (been wanting the Dread Raven for years now) and I got some very annoying errors (the “Sorry, we were unable to complete your payment”). 
After trying over and over, I contacted the online support, which told me to do some various things, like trying other payment options, other browsers, and even trying to purchase while on my phone using mobile data. Nothing worked. 
The last attempt was deleting and re-adding all my payment options, but here is where another problem lies. After doing it, I can no longer add any credit cards. I keep getting a simple but very infuriating message: “An error has occurred.” 
I honestly don’t know what to do anymore. Anyone else experienced this? 
P.S.: Even though my issue has not been fixed yet, special thanks to Agents Sabranduin and Coranyto, for your patience and resourcefulness.