On Steam, I had to exit and launch the game again to register the successful Tome 1 upgrade (Tome 0 was upgraded instantly back then). I kept clicking on Purchase and it kept giving me cheerful error "Transaction was successfully canceled". If I didn't notice the actual funds decrease, it would probably charge me few more times if I had extra funds.
Then I went to the history of purchases and saw the red text of error, telling me that the upgrade was successful but Steam was unable to connect to ESO server for confirmation. Restarting the game to see what happens was the only idea I got from that error ^^'