Pirate Ship offers great USPS discounts—up to 87% off—and easy-to-use tools,...

https://uniform-wiki.win/index.php/Shippo_vs_Pirate_Ship:_Which_One_Has_Better_Integrations_for_Marketplaces%3F

Pirate Ship offers great USPS discounts—up to 87% off—and easy-to-use tools, making it perfect for sellers who mainly ship domestically. Their support is email-only, which might slow things down if you need quick help