Access the underlying GeoJSON service directly via HTTP:
curl -X POST http://conv.sfgeo.org/_shp2json --data-binary @yourfile.shp
Or use sfgeo.org's proprietary temporary WebHook implementation, WebBoomerang, and pipe the JSON to some other endpoint like so:
curl -X POST http://conv.sfgeo.org/_shp2json --data-binary @shape.zip > /tmp/converted.json && \
curl -X POST http://your.iriscouch.com/xcb -H "Content-Type: application/json" \
--data-binary @/tmp/converted.json