Error Code i2501 in the Store

This is really bugging me.

I think it's because I'm sitting on 25 days left of free time granted from registering Stormblood (30 days) and reaching level 30 (15 days).

I want to purchase the Monk level 70 boost, because my Road to 70 is gone, and Monk is one of the two combat jobs I haven't used at all. With the added benefit that my used combat jobs would benefit from the Armoury Bonus, since they are all 64 or less right now.

But it gives me that i2501 error after I click my Payment method.

I know it's not a billing issue, as some say. I never even entered the card info. I literally click American Express and it instantly pops up that error.

That's why I think it's because, while I am registered and not on the free trial, I'm still technically not paying a monthly free.

And the sale on the Monk boost ends before my 25 days are up.