No option for USPS small flat rate box from drop down list on Ship Rush for Stamps.com. USPS rate should be $5 flat rate but our options are limited to: Parcel, Letter, Large Envelope/Flat, Large Parcel, Flat Rate Envelope, Medium Flat Rate Box, Large Flat Rate Box. With Parcel rate, we overpay and with Flat Rate Envelope we underpay.
Running ShipRush for Stamps ver. 18.104.22.168478