I have done some further testing of this and I have been able to reproduce and overcome the problem within the PowerStore. What I noticed was that the parameters did not seem to be matching when I was changing them manually in the store cart file where the cart is created. This lead me to look at the way the cart function was defined. This occurs at the top of this store cart page. I then checked into the eCart definition file and found that the definition there differs from the one in the store cart. When I updated this with the latest version of eCart I was able to create a new cart definition file that had the missing parameter.
I'm posting a copy of my new cart definition file for you. Please backup your current file with the same name then save this one into your WA_eCart folder. Let us know the result you get when you use this file.