Agamic,
I just looked at your website and the Edit Cart seemed to function correctly.
I selected a plain tarp with no color choice and was able to add it to the cart and then edit it (different size).
Then I selected the "striped tarps", selected the size and did NOT select the color. I got the expected error message and REMAINED ON THE PRODUCT PAGE. I selected the color and it went to the cart as expected. Then I was able to edit BOTH the size and the color without problem.
Looks like you got it solved.

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