Post Office lingo: RETAIL is what you pay if you put stamps on the parcel or pay the clerk at the post office. COMMERCIAL is what you pay if you generate via your computer your own shipping/postage labels, like via PayPal.

Near the top of this thread, I showed the zones and payments at RETAIL. Here below are the COMMERCIAL rates/zones:

All found in Notice 123:
https://pe.usps.com/cpim/ftp/manuals/dmm300/Notice123.pdf

Robt.