You would add the columns to the Orders database table not the Products table.
Then you would add the columns to the checkout page: webassist/plugins/shopping_cart/checkout_form.php
Then update the store cart summary in database server behavior found on: webassist/plugins/shopping_cart/process_transaction.php
I think those are the steps.
When you add the datepicker to the checkout form you can set it to require it to be any number of days in the future and restrict days and dates using the Datepicker ui: https://jqueryui.com/datepicker/
Did this help? Tips are appreciated...
