I've been selling a few things on e-bay and have found the same problems with DC's. Plus our post office is always crowded and everything I've shipped with them has gotten damaged in some way. I have a feeling that it's just my branch though. So I avoid it like the plague.
Now I go to the UPS store. Nothing ever broke with them...yet... and I get a tracking number for just shipping ground. Might be a little more expensive, but I find it's worth the peace of mind (esp. if you have to go to my local post office!) ^_^.
Something I've been doing though is scanning the parcel shipping order slip and e-mailing it to the buyer. This gives them all the possible information on their package and lets them know I'm on top of things. Perhaps this could stand in for a DC number if there are problems attaining one? It proves that you sent it and does give a shipping order number that I believe you can call and ask about if there are no other tracking numbers. Might be good for swaps too...
I could be wrong though. I'm just a newbie here :-p