Here's the deal with United Airlines (I flew over on them a couple weeks ago)... I even called to have this clarified as it was really confusing...
If you purchased your ticket before September 30th, you are allowed 2 free checked bags.
If you purchased your ticket after September 30th, however are flying before December 15th, you are allowed 2 checked bags for free. Flying after December 15th you are only allowed 1 for free. Second bag is $50.
Again, with flying United, if you are over the weight limit, baggage too big, etc. the extra charge is $200.
Shipping UPS is ridiculous- for kicks I looked it up before moving. And depending on where you are, to ship a 20 lb box could cost well over $300.
The US Postal Service does offer M Bags. If you have books you want to bring over (and it can be only books), again depending on where you are, a 20 lb box of books will cost roughly $60 to ship.
I know you didn't ask for this info, but you never know- it may be useful.

What I did was packed my monster suitcase (ended up weighing 43 lbs) and found the largest box I could that was still under the size restrictions (ending up being 52 lbs, but they let it slide, luckily!!!). I had one carry-on suitcase and my "personal item" was the largest handbag looking thing that I could find that I jammed full.
Best of luck with moving!
