Purchase Problems/Errors

Hi,I'm trying to buy the 800/900.With Microsoft Internet Explorer, I just can't add the 800/900 to my shopping cart. I'm using version 6.0, win xp. I can add the base 737 NG and the B1900D, but not the 800.I tried the same thing with mozilla firefox, but have just been getting credit card errors. It's not mentioned in the purchase page, but on the authorization page, you don't accept american express?I tried a mastercard and a visa but keep getting "The following errors have occurred.(98) This transaction cannot be accepted."I am trying to purchase it with a visa/mastercard issued in Mexico.Thanks,Alejandro Velasco