Pass a variable to the Invoice

Hi I have a delivery date field (CAT_Custom_295523) set up on my buyer checkout registration page , and I want to pass the selected date onto the invoice so the purchaser sees this as well other delivery info. I tried using {module_url,CAT_Custom_295523} on the invoice layout, but didn't work.

https://happy1florists.worldsecuresystems.com/aaainterim/6-red-roses-i n-mini-box

It works ok on the Workflow Notification received by the seller, but needs to be on the invoice. Any help or suggestions greatly appreciated. Thanks in advance