The trading skill is calculated for the merc you use when talking to the trader, not the others in your team.
The Total Commander plugin works fine if you follow the instructions to the letter. Hundreds of people have managed to get it to work and if it doesn't work for you there's something you've missed. But you can do it in the cammand prompt too, the syntax is something like "AZP x file.azp c:\folder" depending on where and which file you plan to unpack. Just make sure the AZP.exe and archive is in the folder you run the command from. Easy piecy
