I got this today in an email. I'll use it to judge shipping charges from all online shops, including Ebay.
Some examples of orders that can be shipped in a flat rate envelope
4 Charm Packs
A small ruler + 3 Charm Packs + 2 Patterns
1 Jelly Roll + 2 Charm Packs
1 Layer Cake (folded in half) + 1 Charm Pack
3 Yards of Fabric