the Debugging eCart Transaction Failed issues is not specific to Protx or Sage Pay. it is for all of the gateways supported by eCart.
There are a gazillion and one reasons for a transaction to fail, only a small handful of those reasons have to do with a problem in eCart code.
We do test our PDF instructions and they have been successful, if there is a problem, we are more than willing to fix it.
If you follow the instructions for getting the response back from sage Pay and need help deciphering wha tit means, please post back and we will help whatever way we can.