Free Shipping to selected Zips; Shipping discounts to others

[beefbuilder]

I offer free shipping to customers who order $175 or more, and exclude non-ground shipping zips (our products are perishable with a required shipping time of 3 days or less). I would like to extend a shipping break to these excluded zips of $20 for the $175 (or more) order amounts. I thought about a coupon, but then the guys with ground shipping who qualify for the free ship would be able to use it and compound their savings. I don't want to get rid of the free shipping that I have in place, because customers like that more than a straight discount (folks hate shipping costs, and even a discount will not quell their displeasure fully with paying shipping). And an across the board free shipping over all zips would be cost prohibitive to us as non-ground shipping zips are as much as 3x more $ than grounds. Any ideas?