What's the most cost-effective USPS shipping option for high volume orders?

We had a crazy good flash sale, and I have over 25,000 orders to fulfill and ship USPS over the next week. Items will go out in poly mailers weighing between 8oz and 1.5lbs each.

Typically, I would ship using Shopify’s postage feature, however, with this kind of volume, I wanted to reach out to get the community’s input on the most cost-effective postage out there.

I know about these options:

  • Shopify postage
  • SKUlabs plugin
  • PirateShip plugin

What plugin/service would you recommend using for a very large volume of orders shipping USPS?