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 Shopping Cart section of a gallery's Settings.
You can control the shipping speeds that your client has access to in the Price Sheet Settings area of your price sheet under the Price Sheet Settings - Shipping section. If you are using a lab fulfilled price sheet you can choose to activate the lab cost shipping. All price sheets will have access to create custom shipping prices.