// .... find this line
const selectedVariant = productJson.variants.find(variant => variant.id === variantId)

// ... then paste this right below
const discountPercentage = productJson.discount_percentage || 0;
var selectedVariant_compare_at_price = selectedVariant.compare_at_price;
var selectedVariant_price = selectedVariant.price;

if(discountPercentage > 0.1){
    selectedVariant_price = selectedVariant.price * (1.0 - discountPercentage);
    selectedVariant_compare_at_price = selectedVariant.price;

    if(selectedVariant.compare_at_price > selectedVariant_compare_at_price){
        selectedVariant_compare_at_price = selectedVariant.compare_at_price;
    }
}

//... replace selectedVariant.price with selectedVariant_price, and replace selectedVariant_compare_price