To assign incoterm to an EasyPost shipment, create a shipping rule by navigating to the Shipping Rules page and selecting Add rule.
You may set the order filtering criteria as desired, such as when the buyer requests a particular method or lives in a certain country. With that said, you might find it useful to use the "Shipping method requested" criteria, particularly if the requested method describes the kind of incoterm the recipient requested.
Then, under Action dropdown, select "Set customer incoterm (Easypost)" and select the desired incoterm. You will need to repeat this process for each type of incoterm you offer to your customers, creating at least one separate rule for each type.
Note: according to Easypost, "Setting [incoterm] to anything other than "DDP" will pass the cost and responsibility of duties on to the recipient of the package(s), as specified by Incoterms rules."