If on a long journey it's actually possible to ask the operator to order something on especially for you when it's restocked, then that's rather handy!
It's more as a reactive thing - person asks for something I've sold out of / not got their preferred flavour, and, after explaining the situation and offering to order it, I phone the next shorebase and voila, half an hour later, happy passenger. It's not hard, it's not a chore and everyone's happy - I get the sale and the passenger gets their sandwich / red wine etc.