I don't think that they addressed that too much but i've been looking into it and Miva definitely doesn't have a good solution. The best thing we've found out there is using the Miva Module by Vikingcoders.com - and using their inventory manager -
http://www.vikingcoders.com/store/me...BROK_INVENTORY
Still a bit rudimentary as it doesn't allow you to have post multiple items when it comes to clothing as in shirt size and color but we've worked around it a little bit. Check out the modified.com store where you order clothing.