Commerce: How to implement/save 'Use shipping for billing' field for future orders There is a 'Use shipping for billing' field in the Craft Commerce sample templates but it is unclear to me how to use that in the CP or for an account user in their account or for a future checkout.

I cannot find any documentation regarding this field in the Commerce documents. Is this only being used on the sample templates to handle front end display logic?

We want to be able to save this field so that when an account holder logs into their account, we can show their Shipping and Billing address in the correct state (ie. Billing address fields hidden and 'use shipping' checked). This would also apply to future checkouts too.

Is there a way to do this with the Commerce address fields? Or would we have to add a non-Commerce custom field to Users to save this and then add logic accordingly based on that field? And if yes, would that mean a separate ajax post to save that non-Commerce field during a checkout - or would it get saved to the User profile along with everything else?
We achieve this by simply checking the ids of the addresses set on the cart.  Commerce saves the last used shipping and billing address for customers, so you can just use some simple template logic to achieve what you want I think:

        {% set sameAddress = true %}
        {% if cart.shippingAddress and cart.billingAddress %}
            {% if cart.shippingAddress.id != cart.billingAddress.id %}
                {% set sameAddress = false %}
            {% endif %}
        {% endif %}

        &lt;div class="same-address-checkbox"&gt;
            {{ forms.inputCheckbox({
                type: 'checkbox',
                id: 'sameAddress',
                name: 'sameAddress',
                checked: sameAddress,
                value: '1',
                label: 'Use same address for billing address?',
            }) }}
        &lt;/div&gt;


One caveat here, if you offer a checkout during registration, you'llw ant to use my plugin for this as otherwise these last used addresses are not copied to the newly created Craft user - https://github.com/bossanova808/CommerceRegisterOnCheckout