Any idea why a login request would return set-cookie headers that remove the promo code? I can’t seem to track it down.

This is really a shot in the dark and don't be surprised if it doesn't work, but... There's a checkbox that is "PASS URL PROMOTION CODE TO CHECKOUT" in setup website -> shopping tab.

@Timothy Did this work?

Unfortunately it didn’t work. I’m not sure what drives the cookies so I came up with a temporary solution. I grab the promo code from the session before the login function is called and then set it again right after it’s called.