Patch to restrict Media Mail by Product

[dtakle]

Please see attachment.
This patch correctly modifies the USPS shipping logic so "Media Mail" only shows if every item in the cart qualifies.
I've added one field to the products file. You can set it to default either way, depending on what most of your products are. You will have to set the rest manually.

I've seen other people who needed this, and I finally worked it all out. You are welcome to have it "as is".
Please note that I did not separate out he language pieces. All visible items will show in English.