Any packages to Quartzsite can be troublesome, but the best way is through Quiet Times. They provide an address that "should" work for mail, UPS & FexEx:
Your Name
C/O Quiet Times
90 E. Main St - Box 2813
Quartzsite, AZ 85346
The only time this didn't work for me was an air express delivery from China. They chopped up the address & who knows where it went the first time.
Quiet Times charges anywhere from $2.00 to $7.50 to pick up your package, depending on size. They are a couple of buildings east of AZ 95 on the north side of the Main Street - very limited parking.
General Delivery at the post office is a bad choice. One window that is only open from 11:00 - 1:30. Closes immediately at 1:30 no matter how many are in line. If I was going to do general delivery, I'd have it sent to Blythe & drive 20 miles to pick it up.