If you're trying to compare a variable product that's price changes, according to, for example, size or color, you can add the product to retail x by manually getting the URL.
To do this, select the variation you'd like to compare, add the product to cart, view the cart, And then click on the product from the cart.
And here I can see that the attribute is added to the URL and this URL is specific to the product I've selected.
So you can copy this URL and put it into RetailX to be compared.