When I was reselling used books, USPS was the best ROI for shipping (packages and media).
Even before then it was the clear winner. Flat rate standardized (free) packaging is such an awesome perk. USPS loses way too many of my parcels, but it's just not worth going to UPS/FedEx and paying 2x the rate plus way more hassle for a slightly reduced lost package rate.
Of course you know what they say about the plural of anecdotes.
Shipping is highly localized so options are always appreciated, a bad USPS delivery person or a bad UPS person can be all the difference.
That having been said -- because of the discounts offered to large shippers, the non-preferred carrier may cost 2x or 3x as much. For example, when Newegg switched from Fedex to UPS, they continued to offer Fedex. But they passed along the difference in price.
And I should note that UPS loses a ton of packages too. I shipped 50 laptops once and 2 went mysteriously missing (aka stolen).