USPS module, disabling a service for certain weights

I thought everything would work easy by just using the USPS module and letting people pay the actual cost, but there is one problem. I have parcel post, priority, and express selected. I have a bunch of light items that should sell the most, and typically they are going to weigh a pound or two and need to send them priority because they fit in the USPS boxes. The problem here is that I don't want people to select parcel post for those, and I don't know if that can be disabled. I still need to leave parcel post as one of my options though for the heavier items I sell. Does anyone have any ideas how I can work around this?

Well I thought of something that might work. I could do zone rates for the US and just have a couple of flat rights for parcel post. Then I could just offer Priority and Express plus the international stuff. So anyone in the US it would show up as flat rate (which would be cheapest), Priority, and Express, and they could pick what they want. Does that sound like it would work or are there any better ideas?