Use this JavaScript snippet to make sure at least one shipping method is selected if shipping is enabled when we use the AddOns as shipping selector

addons_as_shipping.js

/**
 * Making sure at least one shipping method is selected if shipping is enabled
 */
if(typeof Kr !== 'undefined' && typeof Kr.Listing !== 'undefined' && Kr.Listing !== null && document.querySelectorAll('input[name="addons"]').length > 0)
{
    let checkIfShippingHasBeenSelected = function() {
        if(document.querySelectorAll('input[name="addons"]:checked').length === 0) {
            [].forEach.call(document.querySelectorAll('input[name="addons"]:first-child'), function (el) {el.checked = true});           
        }
    }
    checkIfShippingHasBeenSelected();
}