Hello develop team of Shipstation. USPS have Flat rate (that is really good for shipping). Fedex after that release One Rate service (yes, that is supported on Shipstation)
And why you forget UPS? UPS have Simple Rate (this service have been over 2 years already). With this rate. We have a lot of room for shipment with great rate (can use custom package and only follow the Box Dimensions from UPS). Check it out by this link https://www.easyship.com/blog/the-guide-to-ups-simple-rate-aka-flat-rate?fbclid=IwAR2wvbv9wTShQQyog1i1ruvyaYg90oPLF5PANKThor1OsXX-_odDyTw6LJQ
==> Easyship have a whole topic to explain and tell the rate for UPS Simple Rate. That is very clear and easy understand. All you can see that rate super good. I have to go direct UPS account to use simple rate from UPS and I don't know why Shipstation have One Rate for Fedex but why can't you have Simple Rate for UPS?
Please make us easy to ship on your Shipstaion. Please add Simple Rate for UPS. PLEASE
