Do you mean to pay off a debt or are you just talking about paying for membership?
If you are referring to paying off a debt:
You need to message Billing Support if you want to negotiate payments but the account will stay out of the game until the debt is fully paid.
Use the 'Contact Us' button on the
Settle an Unpaid Balance
page to navigate to a message form.
If you are referring to paying for membership:
There is no system to pay other than the regular payment methods but there is plenty of choice.
The smallest payment interval that can be purchased is 14 days - bought by redeeming a Bond in-game. Bonds can be purchased from the website with real world money or in-game with gp.
The cheapest way to get membership is to earn enough gp in-game to buy and redeem a Bond every two weeks. Earning gp and buying and redeeming Bonds for membership means membership is free.
The next cheapest way is to sub for a full twelve months of membership at once. The biggest discount is applied to 12 month membership.