you're not necessarily running afoul fo federal regulations. last time i tried to ship a firearm with UPS here was several yrs ago now. i had copies of the regs printed out. argued for 15 minutes w/ the UPS clerk before finally his light bulb came on: "OH! i'm not arguing about the LAW! these are our POLICIES!" and their policy was to accept firearms from non-ffl holders to ONLY go to the original manufacturer. more than likely, you're having to deal with a local UPS directors policy. nothing to do w/ federal regs.
for all the badmouthing the USPS gets, i've had abt 800 ebay packages and another 30-40 gunbroker, a fair percentage of those being firearms, and i could not ask for better service than i've had from the US Postal service.
roger