I live in a culture that doesn’t take tips seriously so I may not be a good person to ask, but personally I haven’t come across any true free delivery. “Free delivery” is just the coded word for “reduced delivery fee”. And even then it’s still a hefty amount of money, and to even get that privilege you have to jump through hoops to get the coupon.
The food delivery is much more merciful and I do score “free delivery” often. But then they have the service fee which you can do nothing about and can be really hefty. So sorry, by the time I complete my purchase I’m already too drained out of my money to tip.
If you think the delivery man is doing a good job, then you can tip them as a gift of gratitude. Just don’t spend too much though.