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.
This has nothing to do with the Name or Description field. The first line item is the ID [item Number] as mentioned in the Authorize.net AIM Documentation. If I remove the ID from the XML the confirm page works fine.
According to Authorize.net AIM Documentation the ID field is a string field and by default your cart system uses this ad the primary key of the item from the products tbl. I have a character field called SKU and I am using it as the ID for the Authorize.net XML and I'm still getting the 270 error returned be the AIM API.