[fix] Outdated TypeScript definition for Shipment buy function

[nwithan8]

Description

• Fix outdated TypeScript definition for Shipment buy function, account for EndShipper ID
• Fix unit test for Shipment buy with EndShipper that wasn't passing variables properly

Closes #478

Testing

• Re-recorded cassette for Shipment buy with EndShipper

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)