I'm pretty sure Doordash or at least some of the delivery services let you change your tip. I've definitely read articles about people putting in high tips to "insure prompt service" but then they dramatically drop the tip at the very end (I think they can even do it after delivery).
Uber needs to require the drivers to input the passenger rating before they find out what their tip is, if they want to eliminate the ability for people to "buy" good ratings. (I'm not saying that Uber necessarily wants to do that, but as a "late tipper" myself, I do think it has impacted my rating in the past.)