I'm planning on making it a URL to a pdf of the postage...that work?

I think USPS labels are returned as PNGs - would probably be easier to just shoot that back, no? At least our method of generating USPS labels (Stamps.com API) shoots back PNGs.

Ah, okay. Yes, that'd be easier. Other services provide PDFs but agree PNGs better

You could bake this into the REST API via the "Accept:" header. The default when not specified is whatever you set, but the caller can specify "Accept: image/png" to get a PNG image, "Accept: application/pdf" for PDF, and so on for whatever other types of output you support. The advantage of PDF (or SVG even) is that, if it was constructed using text/vectors, the output resolution is limited only by the capabilities of the printer.

Ah, great idea. I like that