I tried this out last weekend - ordered a few weeks ago for delivery on my London-bound flight. Printed out my receipt (as per the instructions), and after the meal service I asked a member of the crew. Unfortunately that crew member wasn't familiar with the process, but found someone who was.

Purchase came sealed in a tamper resistant see through bag, that was apparently suitable to satisfy security if I was going onwards on a connecting flight.

All in all very efficient, and I'd definitely do it again if there was something I specifically wanted from the HighLife Shop. Although one thing I would say is to make sure you have the confirmation email, and actively ask the crew rather than waiting for it to appear at your seat.