Authorize.net - Please select payment method error

[bwoodsdesign]

Here is my client's site that I'm working on and have been trying to fix this problem for WEEKS. http://www.receiptrolls.com/shop

I've also searched the forums for a solution to this, to no avail, and the possible fixes I did find, were not helpful.

Whenever someone adds something to the cart and checks out, then goes to the page where payment method is chosen, the page shows 2 sets of fields for credit card information. Then, no matter which option is selected and fields completed, Zen Cart returns an error when you click on 'continue'. The error is a red box across the top of the page that says "Please select a payment method for this transaction", even though one is selected.

Can anyone help me figure out the problem and how to fix it? I've tried uninstalling and installing Authorize.net (aim) and Authorize.net modules, back and forth several times, respectively, according to some other users advice here in the forum, but when I only have one of them installed and activated, NO credit card option appears on the shop checkout, and when I have both installed and activated, they both show up but neither one works.

I am using 1.3.6 which I installed to my client's site back in October. Unless you can prove this problem can be solved via an upgrade to 1.3.7 PLEASE I really do not want to get into the hassle of doing that because I have several mods and a custom template installed.

I am also using the Transactions ID and key which were supplied to me by Authorize.net on behalf of my client who is an account holder there, and using it in test mode.

Please note: This was not a problem 2 weeks ago. I have not done anything different to the site files whatsoever except activate from one payment module to the other, within Admin.