I'm going to add a tip to my carryout order myself.
Why are you going to add your own tip?
People don't have the good  manners to add a tip themselves. Sometimes I have to wait for hours to deliver food and it isn't fair not to get a tip.
I see where you're coming from, but a tip is still optional and shouldn't be forced.
I'm going to start tipping myself and see if it works.
Your feelings are understandable. However, messing with the total of a customer's order isn't right. Why not continue to do the best you can at your job so that your customers will want to tip you for your great service?