No need to do it 15 times
You just can't tune a car and send it straight away without saving.
If you make one ZL1, as soon as you do your next race, buy another car or anything that causes an autosave then that car's tune will be saved.
The only time it's a problem is if you go directly from tuning it, hop out of it and send it without returning to the GT Life screen.
Hope that explains it.