When I stop at my favorite tire place for them to check the air in my tires, they charge me nothing. I always give them $5. They always try not to take it and I tell them to use it for donuts in the coffee room. Yesterday, it occurred to me that if they charged me $5 for checking my air, I'd think that was an outrage and I'd likely never go there again and use the gas station air thing and get my hands all dirty and hurt my back.
Ditto with buying software/apps. If I find a very cool free piece of software, I'm very likely to 'donate' $5 or $10. I rarely even download an app that costs $4 or more because it costs too much.
I got no reason for any of that.
I swear I had a third thing but it has now left the building...