AbleCommerce Shopping Cart Software

Is there a way to add a gift message to order after it has been submitted?

You need some customization to accomplish this. All back end support is available all you need is to expose this functionality to Receipt and MyOrder page so that customer can access it after placement. All you need is to make a new page that have the logic to iterate over order items and list all products with a text for gift message and finally a save button at bottom. Once some one saves some gift messages just iterate over shown order items get message text and then put it orderItem.GiftMessage field save the object. Have a look ConLib/GiftWrapPage.ascx and ConLib/Utility/GiftWrapOptions.ascx control about how to process gift messages.