I was hungry once and ordered a lot of stuff from one place at lieferando. I was really looking forward to it and was in a good mood, so I pre tipped through their app. They cancelled my order but kept the tip.
You won't get a bad service if you don't tip and in my student time I rarely tipped, but never had a bad experience with this. Now that I have the money, I usually have a batch of 5€ and 10€ banknotes with me, specifically for tips. I try to tip around 10% and the people usually are really grateful.
Do custom without shame! They are bullying you. These card readers with tip suggestions are getting annoyingly coming here too. One time in Berlin I went to a donut stand and the suggested tips started at 10%. I gave them 1 star on Google maps and complained. They replied that that's just how the machine works. I know perfectly well that's customizable...