I actually got a reply from (I think) a human. In response to my email saying that their first response didn't deal with my question, he/she explained that they send an automated response to whatever appears in the subject heading. The flaw with this of course is that, as far as I can see, when I contact Paypal I can't create my own subject heading but have to choose one of theirs and if it doesn't exactly match my problem the answer won't match my problem either. A classic example of a system set up for the company's convenience rather than the customer's.
Anyway, he/she confirmed that the <removed> text was indeed from Paypal. However they didn't explain why my attempts to text "yes" in response were all noted as undeliverable. And what's interesting is that, all though thy never received an affirmative answer from me, they processed my "suspicious" payment anyway. So I am at a complete loss to explain how this "Security" step actually works.
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