Linden Lab Official:Billing FAQ
General billing issues
American Express
If you're an American Express customer, be happy: American Express is pretty vigilant about guarding your money.
If you're planning to purchase land or L$ through Linden Lab with an American Express card, you should first call American Express and authorize an open billing agreement with Linden Lab. Once American Express has enabled it, you should be able to use your American Express card to buy land or L$.
LindeX™ Exchange
I bought Linden Dollars (L$) but my balance hasn't updated
If you bought Linden Dollars (L$) through the LindeX, they should show up in your balance more or less instantly. So if they don't appear instantly (or after a reasonable wait time), something may be wrong.
As a first step, you may want to check the Second Life Blog to see if we're already aware of the problem and working to fix it. Chances are good that if it's something that affects a large number of Residents, we're already on it.
If it looks like a problem that only affects you, then open a support case.