Add the ability to add gratuity/tip for future orders where payment has already been collected

Some customers take a lot of future orders, and by taking payment upfront they limit themselves in their ability to add tips/gratuity when the customer picks up the order. Currently the only workaround is to leave a future check open as unpaid, but this is not ideal as it increases the risk of food waste. There should be an ability to add a tip to a future order at a later date after initial payment has been taken.