Another thing to do is just buy the subscription codes from some place like Amazon. After purchase, the codes just appear on your screen and you copy and paste them to the swtor website.
A few folks have stated on the support forums that they still pay every time their subscription is due but then remove their credit card from swtor's backend so the card number isn't on file.
One thing not to do is a chargeback. BW's policy is to immediately close out your account and from forum discussions, this is irreversible.
Hope this helps
edit: You didn't say but was the purchase for something in game or elsewhere like online or at some brick and mortar location?
