Looks like you found the cause of the problem?
showthread.php?t=12214
authorize.net can have some pretty strict limitations to the data that is sent, for product info:
Item ID - Up to 31 alphanumeric characters
Item Name - Up to 31 alphanumeric characters
Item Description - Up to 255 alphanumeric characters
Quantity - Up to 2 decimal places, must be a positive number
Item Price - Up to 2 decimal places, must be a positive number