Well this tells me exactly what the problem is... line item 1 is invalid means that you are probably setting the name or description to a field that is too long.... Drag and drop the "Full Request" and "Full Response" onto your page and send those (remove your username, password, and security key where present). Looking at that will tell me the exact problem.
My guess is that either it has an invalid amount or the name and/or description are too long. I can't believe Authorize.net support couldn't tell you that, but make sure to give them the reason code and reason text and they should be able to be more accurate with you.
Or send them the full request that is literally everything you send to them at checkout, and somebody familiar with their api would be able to spot the exact problem.
