Not really Justin.... Theres a couple of ways of doing it.... First is you get slightly less discount (say 15%) and get free p+p etc or, get a larger discount (25% for example) but, get charged an extra pound or two for p+p.....
The last group buy was around 20 - 30 people and the discount worked out about 15% by Paypal, and 17.5% by cheque + free p+p.
People had the choice of a 24x36" Waffle Weave, 16x16" Deluxe Mega Towel, 16x16" Mega Plush Towel and a 16x27" Mega Mega Plush Towel.... So a fair bit of choice as well.
Cheers.