We need to create a Shopify extension for a client.
The extension would need to add a new field to the checkout page (around the shipping address), this would would have to be validated with their js library and would appear under the shipping address on the invoice, packing slip, order detail page etc.
We think we could do this to an individual store using cart attributes but I'm not sure this is possible through an extension. https://help.shopify.com/themes/customization/cart/get-more-information-with-cart-attributes
If you know how to do this, please email firstname.lastname@example.org with examples of your work and your rates as we're happy to pay an experienced Shopify developer to help us with this part if it is indeed possible.