ZenCart 1.3.8 / Have latest USPS update

I sell DVD's, poster, t-shirts, and heavy motorcycle race gear. The Flat Rate Envelope option on the shipping estimator is great for selling small items internationally, but it's horrible when someone selects it for a 17 pound motorcycle leather race suit. Can't fit that in an Express Mail Envelope! Is there a way to disable USPS Flat Rate Envelopes (Express and Priority) for a specific item?

Maybe it's simple or perhaps there's a module that will allow me to control what shipping options are available per product. I'm lost on this one.

Thanks in advance!