Pirate Ship scores big with simple, free USPS shipping at up to 87% off retail...

https://mike-wiki.win/index.php/Does_Shippo_Have_a_REST_API_I_Can_Use_for_Shipping_Automation%3F

Pirate Ship scores big with simple, free USPS shipping at up to 87% off retail rates, perfect for smaller sellers. But if you need faster customer support than email-only or want DHL Express shipping, Shippo might be your better bet