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cart.add = function(productID, quantity) {
 fireProductInfoEvent(`AddToCart`, productID, quantity);
 oldCartAdd.apply(oldCartAdd, [productID, quantity]);
};

Замените на

 cart.add = function(productID, elem) {
 let quantity = $(elem).find(`.qty-switch__input`).val();
 
 if(typeof(quantity) == `undefined`) quantity = 1;

 fireProductInfoEvent(`AddToCart`, productID, quantity);
 oldCartAdd.apply(oldCartAdd, [productID, elem]);
};