If the padded envelope is thick enough, it counts as "package" rate and the cost is a minimum 3-ounce charge. That works out to $2.32 if you pay at the post office, or $1.93 if you use PayPal or eBay shipping services (where they have secured a discount).

I endorse the idea of using sample packages and taking them to the post office window for a price quote. Keep in mind that if they give you a wrong quote (meaning "too low") your customer may end up with a more vigilant post office at the other end and may be required to pay postage due.

I have never found UPS prices competitive for small packages. If anyone has tips for getting better prices for packages weighing less than a pound, I would be interested, as I ship a lot of eBay packages.