How do I set up my shipment to have a third-party account billed for duties and taxes on international shipments?

The options that EasyPost offers for third-party billing are outlined below. You'd want to use the Payment object within your Shipment options to specify billing for postage.

• TYPE: Supported values are "SENDER", "THIRD_PARTY", "RECEIVER", "COLLECT". Defaults to SENDER.
• ACCOUNT: This is the FedEx or DHL Express account number to be billed. Required for RECEIVER and THIRD_PARTY.
• COUNTRY: This is the country code that the account is based in. Required for THIRD_PARTY.
• POSTAL_CODE: This is the postal code that the account is based in. Required for RECEIVER and THIRD_PARTY.

You'd want to use:

shipment[options][duty_payment_account] 

for FedEx and DHL Express with duties and taxes being billed to a third party, and specify the account number to pay duties, along with the incoterm 'DDP'.

Here is a JSON example of how you'd set that up:

options: {
  payment: {
    type: "THIRD_PARTY",
    account: "123456789",
    country: "US",
    postal_code: "84123"    
  }
  duty_payment_account: "123456789",
  incoterm: "DDP"  
}