Completely reasonable. I got an Instacart order yesterday, and paid 15% tip. That worked out to about $2 per item on average. I think that if I can't afford to pay an extra ~25% for my groceries then I have no business ordering them.
>>Cerium+(OP)
Yep, I figure I'm easily saving that money anyway by not going out to eat, not going out for beer with friends, hardly any driving, and so on. So I figure it's the right thing to share generously as long as I'm in a position to do so.