OK. here is a step by step of the method i use with ShopMI
- start TQIT
- select "Play Custom Game"
- click custom game to start, ShopMI
- click "Import" on char select
- click the "TQX Campaign Characters" tab
- click on the char you want to use
- rename char, i.e. MIShopper (at the very bottom of the import screen)
- click the "Import" button
- click "Start" to begin playing
- when you're done, exit the game and CLOSE TQIT
- start Defiler
- click on your renamed char. MIShopper (Custom IT Quest)
- after char loads and Defiler confirms
- click on Character -> Reset Unique ID <--VERY IMPORTANT
- click on "yes" button in confirmation dialog
- click on Character -> Copy To Main Quest
- click on "yes" button in confirmation dialog
- (there should now be 2 MIShoppers is Defiler's list, (Main IT Quest) & (Custom IT Quest))
- start TQIT
- select "Play Titan Quest"
- cycle through until you find MIShopper
- click "Start" to begin playing
- (char will be at the beginning of the game outside Helos)
- go to Caravan and put you new items in the Transfer Area
- exit game to Main TQIT Menu
- Alt-Tab to Defiler
- click on your renamed char. i.e. MIShopper (Custom IT Quest)
- after char loads and Defiler confirms
- click on Character -> Delete Character (Permanent)
- click on "yes" button in confirmation dialog
- click on your renamed char. i.e. MIShopper (Main IT Quest)
- after char loads and Defiler confirms
- click on Character -> Delete Character (Permanent)
- click on "yes" button in confirmation dialog
- (this is just cleanup to get rid of duped characters)
- close Defiler
- Alt-Tab to TQIT
- start main quest with your main char
- go to nearest Caravan, pick up your goodies, and play on

this method is a little more involved than just switching a char back and forth, but i like the extra comfort factor of knowing my primary char is never actually modified and there is less chance of char loss because of file corruption.
hope this helps