Thanks for your reply.

Just so you understand why only now I'm coming up with this issue is that I am intending to implement the "minimum order" plugin (https://www.zen-cart.com/downloads.php?do=file&id=402) however I have amended it to allow a "small" order to checkout with Store Pickup and only block orders that are being delivered.

Unfortunately it makes my site look a little buggy if they forget to enter the date and they change the shipping method from Delivery to Store Pickup and the warnings keep coming up and the method stays at "delivery".

Or maybe I'm being too pedantic..