Are you using a credit card or a debit card from a bank?
LL will not accept "off the shelf" debit cards available at Best Buy, Walgreens, Walmart, 7/Eleven ad nauseum. The Lab wants to be paid NOW, not in 5-10 days as OTS cards tend to do. I suggest getting a PayPal card. I did find that the temporary PayPal card will work as long as you register it with PayPal. When you purchased the card, you dump $20 on it on top of the purchase price. PayPal now has that $20.
Register online the pertinent info required to activate it and you should be good to go. Get $15 worth of lindens (there is a service charge on LL's behalf). Be advised, I'm speaking from my experience with PayPal, so mileage will vary. A few weeks later, I was mailed a permanent card (with a different account number). I am currently using a bank debit and have no problems getting L$'s. Haven't used the PayPal in a quire awhile.
I'm sure someone will pop in with other suggestions.