title: WooCommerce add delivery date as hidden product in checkout (Third party payment gateway)
body: <p>Im developing an e-commerce site with wordpress and woocommerce 2.1. Im using a third party payment gateway (Klarna Checkout) which doesn't offer the customer to specify a delivery date/time. I need to make this feature available but the problem is that the gateway only sends the order-meta from their form, which doesn't offer a date field, when the order is completed. In other words, the standard checkout fields in woocommerce won't be passed to the final order.</p>
  
  <p>My idea of solution is to add a hidden free product called Time, with variations for different time spans, on the checkout page and automatically add this product(with selected variation) to the cart when a customer selects a variation. The specified time would then show as a ordered product when viewing the order in wordpress. How could i do this?</p>
  
  <p>If anyone has a better idea of how to solve this problem feel free to contribute :)</p>
  
  <p>Thanks!</p>