Have you sold on on-line in the past? Anything at all?
I see sellers shooting their wallet in the foot on postage all the time.
They don't know there are options beyond priority mail, they don't know or care to learn about printing postage on-line. USPS even gives a discount on postage if you print from their website. I use PayPal's software because it has some features the USPS site doesn't, but both include tracking numbers in the cost of the postage. Buying it at the counter is .60 I think...
If you intend to sell online what will you use for packaging for 1st class shipments? carefully recycled cereal boxes won't cut it from a business trying to be professional, and packaging is expensive. So is tape, so is bubblewrap etc.
Might look at Uline, they have pretty good prices.
You can have all the priority packages you want delivered to your home, ordered from USPS online. Don't denude the local PO of packaging for everyone else.