My rule of thumb is, over 2 pounds or over $25, go UPS. Can't beat their built in insurance, I have had 2 claims with them and they were handled quickly and well.
USPS insurance is overpriced and hard to collect on. You fill out a form, send it to the buyer, they fill out their part, send it back to you, -Oh and did we mention you have to wait a LONG time before you can even initiate all this? Then they will probably tell you the item was not packed right.