In my case our post office is always packed and they are slow to get people through. Too many times the USPS calculator has been off when I have taken things to the post office so now I have to get a price directly from the counter to give a quote. That ends up having to deal with two trips to the post office if the buyer does purchase the item. If he doesn't then I have wasted a trip for nothing. All that being said there are members I would ship to anyway.
UPS would be easier in my case but the brokerage fees they charge makes that too expensive for the buyer.