USPS does not offer Tracking Numbers, they offer Delivery Confirmation numbers. This is written on the USPS website and this info is as clear as a bell. Delivery Confirmation Numbers are mainly for the sellers benefit and not the buyers.
Yup. All the shippers have advantages & disadvantages...
USPS does not offer tracking, and it seems to be quite slow in domestic shipping. You have to get Registered, Certified + Return Receipt, or whatever if you want tracking... and for high value items, you WANT tracking, because you don't wan ta situation like yours where the buyer thinks you never shipped the item. However it is very cheap at low weights, and just about the only way to go with intenrational shipments. Domestically, though, it sucks... slooooooooow!
Fedex is much cheaper than USPS at higher weights and much faster. We have had literal overnight shipments with FedEx. Once we sold a $500 sled... sent it out on a Friday, the buyer left glowing positive feedback on a Saturday. Also they have a tracking number with all shipments. The only disadvantage FedEx has is that its insurance policy is horrible, so you're taking quite a risk on certain classes of items (you have to convince them it's not a collectible or whatever).
UPS seems to fall somewhere in between. Haven't used them much.
In conclusion, I'd say the best way for eBay sellers to go is to use FedEx Ground for domestic shipments and to use USPS Registered Mail with international shipments. ALWAYS use Registered Mail with international shipments, trust me on this! You don't want to get negged when your item's held up in Customs for 4 weeks and you have no way of proving you sent the package at all. Get an account with both of them so you can print packing labels and have daily pickups.
Also, it is illegal, period, to send eBay items as a gift. Sooner or later customs will catch you. They'll confiscate the package if you're lucky, and at worst, you'll be going to jail or paying outrageous lawyer fees. Explicitly state that in your auction descriptions.