the error is:
Line item 1 is invalid
this means that there is invalid data (Most likely HTML code) in the product name or descriptions being sent to Authorize.net.
i see in the full request:
2 Horse Ladies Black Long Sleev<|><p>Classic 2 Horse design embroidered in Copper & Gold centered on the front of this comfortable 100% cotton long sleeve tee.</p>
that there are ,p> and ,?p> tags being included with the product description, these should be removed.