I don't mind tipping, but if I'm paying the delivery guy myself, then what the hell is postmates charging me a delivery fee for? I have no idea what I'm paying

I don't mind tipping, but if I'm paying the delivery guy myself, then what the hell is postmates charging me a delivery fee for? I have no idea what I'm paying for and who gets what. It's confusing. So i've been just tipping 3 bucks.