Has anyone determined the least expensive way to return the Quilts for Kids?

I usually send 4 back at a time and I have found that FedEx is the most economical and it comes with free insurance and tracking. In order to get the best rate from FedEx, create an account online at FedEx and print your own labels or go directly to the FedEx facility if you have one locally. Do not use the mailbox stores that ship FedEx and UPS as they have a huge markup on the cost.
You can print your label and drop off the box at any FedEx shipper.

I just sent 4 from my home in CT and it cost less than $10; I tell them at the PO that I wasnt it to go "cheep!" Turned out priority was cheaper than surface!

Did you use the largest priority box?

I used my own box and the postmaster put priority stickers on it.