Good question Chum!

The way I did it will probably not work for you, as it's barely working for me.

I created a free shipping option for orders under $25 or so. Essentially, if their order is under $25 the only option they get for shipping is free.

There is probably a better way, but we were at a time/budget crunch.

Depending on which shipping method you use, you may be able to assign your attributes a lower weight.

Thanks for looking!