Can I send empty AR Pmags by US post office  or do they have to go UPS OR FEDEX

USPS should be fine, I shipped a 33rd Glock mag priority mail to GA last month.

Thanks for the help.  I ordered 4 Lancer mags  vendor sent 4 PMAG window Mage  pmags will not work in my AR rifile at all. I probably will have to pay return shipping I am trying to avoid UPS/FED EX  by going USPS

Your gun wont take pmags? Wtf?

I have a Mossburg MMR Tactical AR15 . I have tried some 8 different MAG PUL Pmags clips. Only one would lock in with out serious pounding on the bottom. regardless of the number of rounds in the magazine.

I contacted Pmag (what a treat), I  Exchanged 5 emails and photos with there tech. His advice was to shave metal off my mag lock on the rifle, and or the Pmag step lock.

I said no way this is a new gun. He said sent it back ... NO way I just got different mags that at worst needed a gentle tap to seat the Mag.  Just because you are number 1 in sales does not mean you are the best (GM RECALLS)

I ordered a set of 4ea  Lancer 5 mags which I know work in the Mossburg from Five Guys Guns in NH. They used Adams Arms to fill the order. Adams Arms sent me 4 ea window Pmags in error,  now I may have to pay to ship them back
or may just sell then dirt cheap to recover my money

Surprisingly a great mag for this rifle is the TAPCO gen 3, cheap but work well. I am 10 miles from TAPCO headquarters in Kennesaw GA but no local dealers can keep them in stock

FOLLOW UP  I just heard for Whitey at Four Guys Guns  They are taking care of my miss shipping problem  with the PMAGS my problem is solved.  Very Good Customer Service. and Personal Attention By Four Guys Guns.com

Ive ordered pmag MAGAZINES from midway and a couple other places and they shipped them through usps, so i dont know why it would be an issue.