All you had to say was it was on back order, and to be honest this whole screed has put me right the fuck off you as a business mate fuckin’ hell

I’d say almost exactly that. Except with the swearing. Or go with “I wasnt having a go, I was requesting an update which you failed to provide but your response says a lot about you as a business. Keep that in mind next time someone requests an update you haven’t provided. I do you the courtesy of sending this privately rather than a google review. Any response other than “thank you” and I’m happy to change that”. Drop that last sentence if you wanna be more passive.