Sometimes this one place gives me $20 of food before closing when I only intended to spend $5 so the upsell kinda hurts

But that upsell is strictly self-inflicted. If it's before closing, I can almost guarantee the staff are just hooking you up because a large amount of that food is just going to go to waste since most restaurants require things to get thrown away (e.g. if it's a deli, some of the meats may have hit their shelf life limit).

I'd just tip what you can comfortably afford and what you feel isn't insulting (e.g. tipping someone like $0.50 on a $20+ order). I always follow the 15-20% rule, and possibly higher in some circumstances. But I don't know, I haven't worked food in 20 years, so maybe the manners/expectations have changed nowadays. Also depends where you live.