Do eBay buyers have a preference on which shipping company you use?
I prefer UPS because the tracking is better. Most of the time it ends up being USPS due to price, they are both about the same distance from work, I go over on my lunch hour to drop the packages off. I charge a flat fee for shipping, I figure my own zip code in NJ and use 90201 (pretty much the farthest shipping point from me) can get a quote and round it up, that's what I use as a flat fee shipping quote for my auctions. Usually it's less and I pocket the few bucks difference. Recently I sold some phones online, I listed $45 for USPS parcel post, but since the buyer was in Ohio, I used UPS instead, which was about the same price to ship there. The nice thing about UPS, is they don't put the price on the label, so I can over charge for shipping and the buyer is none the wiser.
I used to do a lot of international shipping, that was all USPS, I used UPS for my first international shipment, but UPS uses a private company to assess the tariff charges, they charged as much tariff on the item as it sold for on Ebay, needless to say the person receiving the package wasn't happy. Since then I used USPS, but the last shipment I sent to Mexico, was returned to me a month later, undelivered, with items missing (Mexican customs took some). So I'm pretty much done with using the post office for international shipments, too much hassle.