Hello stickfigure,
I checked with the team behind Fulfillment services and they found that Shopify is not rapidly calling any of your callbacks at the moment, but at one point we did attempt to send you about a thousand fulfillment requests between 2020-01-28 21:21:00 UTC and 2020-01-29 19:07:16 UTC. They then think that you updated the callback, after which our systems stopped calling rapidly.
It turns out this is a known issue with a fix in the works. Apologies for the inconvenience!
We appreciate that you’re using this new API, and would love to hear any other feedback you have!
Regards,
Chris