I meant to post this, so I'll put it here as a reply.

First let me say these important things:
¶ virtually all of the postage price choices are found in this great document -- Notice 123. Bookmark it:
http://pe.usps.gov/text/dmm300/Notice123.htm#1118064
The charts below are from this Notice 123.
¶ "Retail" are the prices you pay the clerk at the post office.
¶ "Commercial" is the price you pay by printing postage online, like thru PayPal and eBay. You don't have to make any agreement beforehand with the post office. You get this price even if you print postage for just one parcel.
¶ Note how you get discounts by printing postage online (though not for the first 3 ounces) PLUS you get free tracking by online printing.
¶ "Commercial-Plus" has to do with large shippers that agree with the post office to ship large quantities -- doesn't affect us.

Here is the Retail chart -- for paying the clerk at the post office. Odd (to me) that you can send first class parcels (see red arrow) up to only 13 ounces. Also odd to me that the first few ounces ($2.54) is less than if you print online postage ($2.60, "Commercial"), but you get free tracking if you print your own postage.

Here is the "Commercial" chart -- where you print your own postage (like with eBay, PayPal). You also get FREE tracking with each printed label as well as discounts for virtially every weight class. Note, you can print postage for first class parcels up to 16 ounces (not the old 13 ounce limit) :

Robert