Your client may not understand that although he or she has chosen Overnight Shipping, this does not mean that he or she will have the order tomorrow. You'll want to be sure to communicate to your clients that there is lab processing time, as well as whatever amount of time your studio needs to finalize and approve the order.
Some photographers explain this to their clients in the Optional Message Shown at Checkout area. This message is displayed to your clients at check out and is established within the Prints section of a gallery's Setting.
You may also use the Shipping Title area on your price sheet to specify the additional time needed. This text will be visible in the checkout area.