There was a problem finalizing your purchase. Line item 1 is invalid. Please go back and try again.

I have an Authorize.net account, have verified the ID and key, and went live. Get this error: There was a problem finalizing your purchase. Line item 1 is invalid. Please go back and try again.

I have searched high and low for why this is happening, and have spent a week on resolution. Please help. I spent $79 for this plugin, and it needs to work. I have also gone to Authorize.net's AIM integration help, and followed all the instructions, but it's still not working. What is missing?