I'm having the same problem, and I agree that SL billing customer service is USELESS. I called twice and I followed all of their suggestions with no resolution. They should have offered to research the issue, replicate the problem, but they did not.
On Sunday when trying to buy L$, both inworld and at secondlife.com my payment was declined. I tried a second card on file and it also declined payment. Deleted the cards and have been unable to add any others, and I get the errors listed below. My credit card company is NOT declining them on their end. In fact, a $1 pending payment is visible. This disappers after a couple of days.
I had tier due and was unable to purchase L$ to pay it. But I'm lucky because I can pay the landowner directly with PayPal.
I've kind of had it with SL.