Like a bunch of you guys here, I always get on board for the Kogan mobile deals and convinced my parents to swap over.
When you enter your vouchers you're asked to put in credit card details at the same time.
I set up both my parents accounts and used my credit card, and both times was charged $125 on top of my voucher and had to go through support chat to have it reversed. Just a PSA for anyone who has done the same to check their credit cards to make sure they weren't double charged.
My mums was a fresh account on a new Sim and dads was rolling over his existing plan (had to cut it short 1 month, but still a good deal) if anyone wanted more specifics.
Also the most recent deal vouchers are due to expire end of the month so make sure you don't forget!