suggestions for good USB stick duplication

Hi folks,

I'm looking to sell some software as part of a charity project on USB flash drives sometime next year.

I'm wondering what is the best app to duplicate USB flash drive on a regular PC in say, 4 or 8 at a time.

I'd like to maybe farm out this to someone to do this for me if I get 100s of orders to my product, give them a USB hub and ship new sticks to them from Amazon or whatever.

I think commercial USB suppliers and duplicators will only do this in 1000x pieces or above.

The other snag, is not all USB drives are the same. I have 4 different brand 8Gb sticks and their sizes range from 7.1 to 7.6Gb, so the apps that do a 'forensic' copy of the data can create problems if you want to copy to a stick slightly smaller this won't work.