I have had great luck with priority mail. I have done first class with less sucsess. For all Orders over $10 I do free priority.
My main reason for priority has been reliability. I have shipped first class that was 3 miles away, and for god know what reason it went out on Tues and arrived Friday. There were also a few other similar incidents. Priority mail is nice free supplies(although I have yet to switch to theirs since I have like 800+ of my own supplies still left, gotta love bulk orders). And as for UPS and Fedex, dear god, before I started doing mass shipping I loved them, always feared USPS, but how wrong I was. For next day shipping UPS from Chicago to NY is about 25 for a kit(I think havent price checked this in many many months). Packages going out in morning, I have had Prioirty Mail arrive in 2 days there(Sent monday, arrive Wed). California, 2-3 days. For 4.80 as long as it is under a pound.
My only problem is when I shipped really small items, few batteries, couple packs of cartridges,
eliquid, etc, it seems a waste that I mail something very small in size in a 6x4 envelope and 2-3oz for the same price I ship 16oz large envelop that is 16x11x1. But thats how I decided to go about it and Im happy.
Also I use paypal merchant pro(other merchants for the most part dont allow ecigs), and by doing my shipping via paypal usps, it also allows me to protect my self against "The item didnt arrive" because paypal has the delivery record. But they also do support UPS too.
Here is one example:
Bullet Delivered, May 13, 2009, 12:56 pm, DALY CITY, CA 94014
Bullet Arrival at Unit, May 13, 2009, 5:00 am, DALY CITY, CA 94015
Bullet Processed through Sort Facility, May 12, 2009, 10:32 pm, SAN FRANCISCO, CA 94188
Bullet Processed through Sort Facility, May 11, 2009, 10:49 pm, CHICAGO, IL 60701