This simple code in MWS Scratchpad gives me this 50001 error "The XML you submitted is ill-formed at the Amazon Envelope XML level at (or near) line 2, column 14."
<?xml version="1.0" encoding="utf-8"?>
<messagetype>OrderFulfillment</messagetype>
<message>
<messageid>1</messageid>
<orderfulfillment>
<amazonorderid>171-0326363-1826719</amazonorderid>
<carriername>La Poste</carriername>
<shippertrackingnumber>1K03004689202</shippertrackingnumber>
</orderfulfillment>
</message>