I have sent no less than 20 quilts by the USPS.....with NO problems.
I wrap the quilts in plastic bags --- it's only temporary, but prevents them getting dirty and wet should anything happen. Target sells HUGE zip lock bags for about $1.00 each (in pkg of 5). I use those for customer quilts.
I use the Priority mail boxes -- the flat rate boxes. I've been able to squeeze in finished queen size quilts for about $16.00 per shipment. Be careful, apparently some POs won't allow you to pack till they bulge. However, I just received one yesterday that was taped and bulged to the point of ridiculous, but the PO delivered it!
I've insured for the cost of the fabric and the cost of the quilting -- however, I've never had to file a claim -- so don't know what the PO says. I'll ask next time I'm there, for future reference.