EasyPost is not a 3PL, they are a provider of API access so you can print labels and manifests easier than trying to integrate with each of those carriers' API yourself. We use them here for some of our carrier integrations but we're also not a 3PL.
Sam from EasyPost here. We are a 3PL as well. We offer all inclusive rates that include things like receiving, storage, pick/pack, shipping label anywhere US /Int., warehouse insurance, etc.
Would be great to connect with you. Send me an email at firstname.lastname@example.org
I would stay away from easypost/shipbob/shipwire. They are very expensive for shipping and charge random fees. I'm currently using Librex Fulfillment for all of my 3pl services. They have the best prices I have found. They also don't charge pick/pack fees so all they charge you for is shipping.