On a related note, I worked as a pizza delivery driver years ago, and my expectations varied. If I averaged $2 per place I was pretty happy, but roughly half the people I delivered to paid me less than a dollar. (If I owed them $2.45 back, they'd take the $2, but tell me to keep the 45 cents. So generous!)
Mind you, $2 isn't a great tip, but it added up where I worked, which was pretty busy in the evening. I felt good with a $3 tip, and anything over that was great. I did get paid for gas, but apparently most places pay even less for gas now (7 years later), even though gas prices have gone up.
On another note, I've never really minded when people honestly can't afford to tip. Yes, it'd be nice if they could, but if I got paid in bills and then the last $3 in coins, it's clear that you've scraped up what you can to be able to afford the pizza. Sometimes even poor people need to splurge. It's good for mental health. It makes being poor more bearable in the long run. It gives you a tiny bit of relief, and something to look forward to next time, as sad as that might be. (I say this bit as a guy who lived in his car for 3 years... mind you, it was by choice, and I had a waterbed and wireless internet, but I was still poor.)
Also, if you can't afford much, and indicate that, you always were okay with me. Someone giving a $1 tip and saying "I know that doesn't go far, but..." and then trailing off will get a pass. You gave what you could; you get my thanks.
And children always got a pass on tipping. I just didn't expect them to know that they were supposed to. If someone between the ages of 8 and 15 answered the door, I figured that there was no tip, and I wouldn't get too upset by it. And people might be surprised how often this can happen. (It's not super often, but I wouldn't consider it rare, either.)
The people that I disliked most when it came to tipping were the people that gave me the coins back as a tip, the people that didn't tip (these two made up roughly 50% of the people I delivered to), and the people that literally said "I'll get you next time." Yeah, sure you will.
Overall, though, some people can be amazingly generous. The people that tip $10+ (those are rare, but they do happen, especially on $90 orders) are awesome, and I'll always remember the specifics on those orders over the "here's $0.08, bye." The awesome tippers out there make up for a lot of bad people, and can really turn your day around, as the guy getting tipped. Not just financially (by making up for other tippers), but they can restore a bit of faith in your local humans on a particularly bad day. So, cheers to you awesome tippers.