Hi John,
It seems to me that the error might be in the order numbers you are sending back.
The packet going back to the server needs to be valid xHTML. If all the "Messages" look right, then that's where I'd concentrate my attention.
So the big question,
What does an "order number" look like in your system. I'm guessing it's not 1,2,3,4 etc
Cheers
Bruce