Simple solution: use flat rate USPS priority boxes. Order a few of each size for free and stick them in your closet with a roll of packing tape.

You know exactly what it will cost, you can get a free next-day pickup from your house, and you can use newspaper for packing material.

Only works if you ship small-ish stuff, but it's brilliantly easy if you sink 10-15 minutes into the prep.