Product price £0 select attribute to get price

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I have products that i want all at £0 then when a customer selects if they want a mattress only then the price is £159 if it a base and mattress £315

I have created two custom fields that are cart variants so when they select on of the items from the drop down that is the price they will pay.

The problem is, is that when the product is £0 the add to cart button and price do not show, how do i get these to show.