Whenever I'm in a store that has mayo packets in the open I just grab a few even if I'm not buying something that they would expect one to use mayo packets with. Some might call this stealing, but the cost of a mayo packet is around 15-20 cents, and I'm more likely to visit and make a purchase for my other needs at an establishment that has free mayo packets so I'd say it's a mutually beneficial relationship.
I'm not really sure, I try not to take so many at once to abuse the privilege of open access to mayo packets because then they might decide to keep them behind the deli counter where you have to ask for them (many stores do this)
I try to take around 2-4 packets per store visit, if I know I'm gonna be making tuna salad soon then I'll up the take to 5-6
