In the Archived documentation section of the eCart support page there is a tutorial for "Creating a Coupon system for your eCart store".
this tutorial walks through adding a text box to the cart page and storing it's value in a session variable. the coupon text box is basically the same idea as your text area.
once you have the text area value stored in a session variable, you can modify the store order summary server behavior on the confirm page to save the session into the order details table.