I have worked as delivery man for 3ish years (2007-2010). had some great memories to it. It was like a hype in my city, so everyone me and my brother chilled with worked at restaurants. Sometimes racing against each other in snowy roads, good days. But we had some people unlike you who didnt tip shit, im fine with that, but on heavy snowy cold nights you should tip someone. That's like an unwritten rule. Thus I salute you, good comrade.
We have VAT in the UK, which is basically the same, but when you see a price in a store, it includes that tax and you don't need to think about it at all.
Oh. I think it's because we are multiplying the total cost of everything you have by a percentage instead of adding. So, I assume you don't have that same situation as us Americans.