Adding "addons" to a product when adding to cart. Without messing up Stock #s

I didn’t know where to post this question, so Im placing it under the main forum.. Hopefully someone can help. Not sure if I need an App for this, or how I can get this to work.

I want to offer a product for sale. It will be a limited edition item.

I want to offer the item to be Signed for an additional cost. and then another offering on top of that.

I want this to be a checkbox, or some kind of additional added cost to the cart. but I want the base item to stay the same, I don’t want to create separate stock or separate products for a signed one, etc. I don’t know how many customers will want one signed vs not etc. so I want to do this seamlessly. I’d prefer to NOT make a product by itself for “add signature” with no stock tracked.. that may confuse customers, or make them realize too late after they checked out and buy it separate.

Creating a variation is a different stock level. I don’t want to over sell or under sell due to people wanting one vs the other more. I can’t read minds, but i want to offer signed for as many as I can.

Please let me know if anyone has this figured out yet! Thank you very much in advance!