Y'all should see the difference in online pricing for Express Mail! I sent a box of fish over to California a few weeks ago and saved myself (and the customer) about $10 just on that one box by printing it online.
Just a thought... if you're shipping enough plants to go up to a medium flat rate, chances are it's going to be cheaper to order the non flat rate boxes online and ship by weight. Doesn't help much if you don't have your own scale, but just thought I'd add that in.
Oh, and the free delivery confirmation service alone is a $.70 savings per box.