Help with shipping via USPS

I ship all my products via USPS. Priority Mail Flat Rate Envelope or Box for domestic orders and Express Mail Flat Rate Envelope or Box for individual orders. I have the USPS module installed and it seems to be working fine except for one thing. My products are pens, shipped in a pen case.

I can fit three pen cases in a Flat Rate envelope. How do I get the USPS module to change the shipping quote from Flat Rate Envelope to Flat Rate Box if a customer orders more than three pens? I have tried to achieve this using weight, setting the weight of each pen at 0.5lb but that is not working so well.

My website is at http://www.syzygypens.com

Thanks for any help :smile:

Anyone? :smile:

I'm not sure, but I would be very interested in the answer. I have a similar problem.