I am very sorry, but when I put that message at the very top of the confirm file, the checkout page refuses to load, and I immediately get a 500 error.
I tried uploading a fresh checkout page and that did not work.
I have reverted back to my backup file. That now seems to be working OK, in the sense that I arrived safely at the checkout page.
I am willing to try again.
The steps would then be (1) to first insert the test coding you just sent, with no other changes occurring, and see what happens;
and then (2) if that goes OK, to then move the relevant coding to beneath the mailing.
Am willing to do this.