An optional stamp label printer can be used to print USPS First-Class Mail postage on small labels without an address. PostalPoint expects standard address labels measuring 1-⅛ × 3-½ inches. In Settings → Printers → Stamp Label Printer, select Brother DK-1201 or Dymo 30252 for this size. Most label brands sell an equivalent size under a different part number, and it will work the same way.
If you use a Brother QL printer for 4×6 shipping labels, you can use the same printer for stamps by selecting "Brother DK-2243". There's no size to configure; the stamp dimensions are built in.
PostalPoint also supports printing stamps on 4x2 inch labels, but this might be too large for some envelopes.
Generic labels cost a fraction of the price of name-brand ones and perform the same, so there's no need to pay for genuine rolls.
You do not need special-purpose stamp labels with special fluorescent pink ink dye on them.