When trying to send a draft, I receive an error:
"ErrorIrresolvableConflict","message":"The send or update operation could not be performed because the change key passed in the request does not match the current change key for the item., Item could not be saved because conflicts were detected when saving.
The message seems to send later when trying in a sandbox.
I fixed this by inserting a timeout between the creation of the draft and sending the draft. Essentially, race condition issues.