the line item information being sent is:
x_line_item=759<|>TEXSPORT Mini-Floating Lantern<|><p>TEXSPORT Mini-Floating Lantern</p><|>1<|>11.99<|>YES
the problem is the <p> tags in the description:
<p>TEXSPORT Mini-Floating Lantern</p>
authorize.net will only except letters and numbers in the line item information, any other characters will cause the line item 1 is invalid error. in your case the issue is caused by the "<", ">" and "/" characters