Just to clarify, are you on USPC or Steam? You can check the payment options for USPC from the onigiri website:
https://onigiri.cyberstep.com/chargeAs for gocashgamecard, all they said was to register and activate, unless you can get it physically from shops. You can contact CS about your email authentication issue using their form:
https://www.cyberstep.com/help/mailform_n/select_type/us_og under Billing.
I have never actually bought OC, but they do tend to respond relatively promptly when it comes to giving them money.