I generally see the voucher presented after the drink is presented, which in my experience is after everyone in the row has ordered. I suggested a card rather than a paper voucher in an attempt to control for this outcome, since I suspect swiping a gift card is interpreted as a substantially similar action to purchasing something outright.

That being said, I doubt the effect is caused by a single factor. For instance, there is a social stigma against drinking alone, at least in the United States. I'll feel more comfortable drinking if someone else is drinking as well, even if I'm travelling alone.