WooCommerce Hidden Product as a cross sale

My shop only has three products visible. I have 1 hidden product that I want as an “add-on” that shows up in the cart for my buyers to click on.

My products are 3 memebership levels and the hidden product is a $10 donation that I want people to be able to add to their cart.

With the product hidden it won’t show up in the cart as an add on.

How can I do this? Woo docs don’t seem to mention it.