Checkout does not work.

I am using Authorize.net payment gateway. I've added my credentials in the global file. I can't get checkout.php to work. It loads the form fine, but no matter what, it will not accept any of my VALID credit cards. In my global file, I have tried putting the testing to True, and false, and in authorize.net section of the global file, I have tried putting the testing sandbox to true and false, and none of those options produce any good results when entering my actual credit card information. always says that it can't do the transaction, try checkout again. (like I put a bad information in). In my authorize.net setup (on their site), I have it in testing mode because I didn't want any charges to actually go through.

I've setup eCart checkout pages before with other custom sites, just not with this powerstore setup. So I do know how to get it to work with other "regular" sites that I'm taking payments for.

I haven't changed anything in the cart behavior or the checkout process other than adding the necessary information into the global file.

What do I need to do to get this to work??