I have shipped a couple of times and used USPS. No hassle, no inspection. If you declare it and let them know you are aware of the regs and the gun is unloaded, no ammunition in the package, (preferably mechanicaly unable to fire, for example, bolt removed) they probably won't bother you.
If you get a hardass, you can let them know that Postal Regs allow a written declaration that the firearm is unloaded as an acceptable alternative to an inspection of the package. It helps to have read the regs and carry a printout if you are worried about it.
Be sure that you are shipping legaly ie. to an FFL if out of state. You cannot ship handguns, an ffl has to do it for you. The postal regs say you must ship according to laws, the ATF FAQ is a good guide for this and I would reccomend a printout of this as well to answer any challenges.
If you are in a larger town, the main post office may have a seperate business counter for bulk mailings and other business mailing seperate from the regular counter. You might find that a better way to go.