Remove all credit cards.
Update your billing information.
Did you move addresses: Yes? -> Make sure to contact your bank so they know. Billing information must match the card used.
Add the credit card/debit -> sign out of the Mogstation and/or Square-Enix account info -> Sign back into mog station (from closed window) -> Reprocess the retainer on the site under the additional options.
The first two retainers you get automatically with your subscription. You also have to be up to a certain point in the MSQ to obtain them (ifrit I think normal msq version).
All other retainers cost additional $1-2 dollars per 3rd retainer and higher, but also requires a difference in fees for purchasing them if any sub time is active + newly added to cover the difference for the whole sub time.