How do I add a line item with price at checkout for personalization options?

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I'm using Supply Theme & would like to offer general add-ons for personalization (not by product or collection). For example, if customer would like to make it a Gift Basket, I offer that option for a flat fee which I would like the option to display at checkout. I would prefer to not use an app if possible as I don't feel its cost effective for what I need.