Addon Can't be Bought alone - Help

[1Cup]

Hello All, 

I need to create an addon for a prodcut that could only be sold with another prodcut. 

I found an app that can add an addon prodcut but that addon can be bought separately and I don't want that.

The reason why I need this option is becuase I have some prodcuts that are low in cost and if they order it with free shipping we get no profit at all due to shipping cost.

Any recommendation please?