there are a lot of good arguments against tipping culture (and some for), but it's really not that hard to figure out how much to tip a delivery driver. in normal times, anything at or above 15% (or $2-3, if it's a small order) is enough not to get remembered as a bad tipper.
if you're in an unfamiliar situation, you can always just ask what a typical tip is. service staff at a fancy establishment will probably try to evade the question, but otherwise the person will usually answer honestly.
if you really just want to "set it and forget it", there's virtually no situation where someone would hold a 20% tip against you.
aside from the fact that it's custom and the prices/wages are already set with the expectation of tips (ie, adjusted down), you are essentially bidding for attention. stiff the delivery person enough times, and you might find that your future orders find their way to the end of the delivery route.
if you're this concerned about saving money, you probably shouldn't be ordering food for delivery or dining out in the first place.
if you're in an unfamiliar situation, you can always just ask what a typical tip is. service staff at a fancy establishment will probably try to evade the question, but otherwise the person will usually answer honestly.
if you really just want to "set it and forget it", there's virtually no situation where someone would hold a 20% tip against you.