[Plugin: eStore] Resctrict ‘Add to cart’ to only registered user

Hi to everyone.
I would like to know how I can restrict the use of “Add to cart” button to only registered user, in order I can be sure that non-member users could not perform any shopping without login or register.

I am thinking to add an “IF” check in the function that controls the “Add to cart” button, something like this:

if is_user_logged_in {do the normal routine}
else {redirect to login-register page}

but I couldn’t find where in eShop plugin is stored this button action.

Could you please give me some advice?