Offer free item with purchase of x dollars?

I sell decals and just starting to sell t-shirts. I am wanting to do a Christmas promotion where I give away a free t-shirt (of the customers choosing) with x dollars of decals purchased (don't know the amount yet). I was hoping I could do this with a coupon code but don't know the best way to set it up. Is it possible without mods? Any suggestions appreciated!

-Jeff

Did you have any luck with this? This is exactly the thing I want to do.