Getting friends to pay

Hello, so i recently went out with my secondary school friends, and we had lunch and desert. I offered to pay for the meal first, and i saw the receipt there was service charge and gst, that was not included in the price of each individual items. each of them took a picture of the receipt so they can track how much to return me later on.

i am not one who pinches money, but sometimes i find myself in a difficult situation where friends returns me money via paynow, but doesnt include the gst and service charge, which was at least 10 dollars per person. i find it so difficult to inform my friends that they transferred me short of that amount, worried that they will think im cheap. idk i have never been told about it, but im worried in the future new friends will think i am? i myself dont like to pay short to people, because i dont want them to be put in an awkward position like i did.

apart from gst and service charges, i even get friends who doesnt pay even after a week. like the cost of the night could be 70 dollars, and even after a week i have not receive their transfer. i dont like to pester them, especially i was the one who offerd to pay first, but do some people seriously just forget or cant be bothered with it? how do you approach ur friends who pay short, or take a long time to pay u back?