I ship about 1,000 packages a month via USPS - 99% of them get where they are going - and we, for the most part, do not purchase insurance. We ship a small amount via UPS - but only because we have many clients in the northeast and if we use UPS ground within the tristate area it functions as 'next day delivery'.
My beef with UPS: charging extra for residential delivery, and fuel surcharges, and all kinds of other charges that you don't see up front. As well, internationally, which for us with UPS would be canada, customs is a mystery, and charges a base 'surcharge' of $30 (in some places) just to get processed. Otherwise we would use them more.
FedEx: Just won't do it - too unreliable. I have many vendors who ship to me via FEDEX and they are satisfied with it - but they are shipping to BUSINESEES mostly who are always open - I would not use fedex for customer delivery
USPS - we process all our payments via paypal, and ship 90% straight from their multiorder system which allows you to produce labels for multiple packages at once. The pricing is good - there is a slight incentive for paypal users and it provides good tracking. We have found that, aside for the irritability of postal workers, which seems to be part and parcel (no pun intended) of being a postal worker, they are fantastic. Both Paypal and Stamps.com (we use both) work great.
BOth Stamps and Paypal integrate with many 3rd party shopping carts so, for instance, we don't have to type anything into either, the orders download on their own and you just put the weight in and produce the label. Stamps.com can be a bear to configure, especially if you want to use their SCALE to automatically preload weights, which we do.