Yep... think about it, have you ever see a Postal Service plane? We use Fed-ex, UPS, Delta, American, Continental, etc.
That's why I laugh when the media mentions the percentage of total operating costs that goes to payroll compared to the other two major players... I'd bet it's pretty costly to maintain your own fleet of planes...
USPS also DELIVERS packages for Fed-ex AND UPS to the addresses they do not service...
I would definitely not use USPS for shipping items of length (rods) for cost and (sadly) other reasons.
A box of plugs going further than the mid-atlantic will usually be MUCH faster for the same price using USPS priority vs. UPS ground.