The post office clerks don't have to think much or remember much or look at a chart -- they simply plug the mailpiece weight, etc., into their computer to produce the rate, postage label, etc. If we do the same (example, with with Paypal "U.S. Postal Service - Create Your Shipping Label" -- https://www.paypal.com/?cmd=_ship-now ), whatever we get produced will be legal and acceptable. It looks like there are EXCEPTIONS/MODIFICATIONS to what I thought were two either/or rate charts: "Retail" (what you paid the clerk at the post office) and "Commercial" (what you paid online to create your own postage labels).

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Here is the Retail rate chart from Notice 123. Notice it goes up to only 13 ounces. Any higher, you must use Priority Mail or whatever, but not First Class Parcel postage:

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Here is the Commercial rate chart from Notice 123. Notice it goes up to 15.99 ounces:

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I just went to PayPal's "U.S. Postal Service - Create Your Shipping Label" ( https://www.paypal.com/?cmd=_ship-now ) and I got these surprises (exceptions or modifications of the Notice 123 tables above):
¶ I could only create a shipping label, for First Class Packages, for parcels up to 13 ounces. I couldn't do it for 14, 15, 16, etc., ounces.
¶ I got the same postage rates as in the "Commercial" chart above except that the 1, 2 and 3 ounce parcels are $2.54, not $2.60.

I wonder if I would get the same results with an eBay transaction.

Robert