Understand and implement Stamps.com / Endicia postage label concepts via API for domestic USPS mail classes

Verified steps

1. Understand the platform context: Stamps.com and Endicia offer postage APIs that allow certified developers to purchase USPS postage programmatically; access requires API certification and a Stamps.com or Endicia developer account — contact their developer programs for access (verify current onboarding process as it may have changed with Stamps.com's acquisition by Thales/Neopost/Quadient).
2. Authenticate using the credentials and authentication scheme provided upon API certification (legacy Stamps.com API used SOAP/XML with a username, password, and integration ID; verify whether a REST API equivalent is now available in current developer docs).
3. To purchase postage, construct a label request specifying mail class (e.g., 'PM' for Priority Mail, 'FC' for First-Class; verify current code mappings), package dimensions and weight, sender and recipient addresses, and the desired label image format.
4. Submit the request to the postage label endpoint; a successful response returns the label image (typically base64-encoded), a tracking/USPS IMpb barcode number, and a postage amount; verify the exact response structure in current docs.
5. Print the label on a compatible label printer; USPS requires Intelligent Mail Parcel Barcode (IMpb) compliance which these APIs handle automatically when generating labels.
6. Void unused labels within the carrier-allowed void window (verify with Stamps.com/Endicia; USPS typically requires void within a set number of days) to recover postage credit.