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This sounds like way more trouble than the $50 could possibly be worth.
Send your best contact there a screenshot of the email, saying that while you appreciate the gesture, your bigger concern was making sure they hadn't been breached.
Whatever happens from there will probably work.
It definitely wasn't a breach. But yeah, ultimately, I never had any intention of accepting it, for the reason you laid out. It's $50 against a good, low maintenance client who has already added work beyond our initial project. Never in doubt for me, but a buddy I brought it up to (in the same industry) said I was paranoid. Talking $50 against a revenue stream with several more zeroes than that. Obvious pass.
I didn't bring it up to my point of contact, because I doubt anyone cares either way. It's a California tech company that probably spends the same amount per day on catered lunches and the barista in their lobby that the entire gift card spend cost. Basically just marked the email read and moved on.