The smaller Flat-Rate Envelope is $4.75 if the postage is paid online (20-cent discount), but for some reason, you don't get a discount if you select the legal-sized envelope.


This is copied from the postcalc site (the first price is for paid at the post office; the second is for paid online):

Priority Mail® Flat Rate Envelope
USPS-Produced Envelope: 12-1/2" x 9-1/2" $4.95 $4.75

Priority Mail® Legal Flat Rate Envelope
USPS-Produced Envelope: 15" x 9-1/2" $4.95 $4.95


I just now opened the page to print postage for an eBay sale, and even though it's going to go First Class, I selected both the letter-sized Flat-Rate envelope ($4.75) and then the legal-sized ($4.95), so yup, I'm sure. Next opportunity you get, do the same thing and see if you get different results. I'm at a page that starts https://shiptrack.ebay.com.

Sue