ZC 1.3.8a

I have a product which has numerous sizes - each of which is priced by its options. We want to offer discounts for purchase of 10 or more - but they can be "mix 'n match" among the sizes.


example:
10 or more = 20% off

Shopper adds these to cart:
3 of Product "A" - option "size: small - $13.00"
2 of Product "A" - option "size: large - $15.00"
5 of Product "A" - option "size: medium - $14.50"

The above totals a qty of 10 and should qualify for the discount ...but so far my Zen Cart is treating them like separate prods. Is there a means to allow the above discounting scheme??