Adding line item on packing slip - Rental booking period

[cupio-closet]

Hello,

I own a rental booking site and I am wanting to add a line item to my customer's packing slip and invoice that includes their rental booking dates. I'm using the app 'BookThatApp' as my rental system. Does anyone know what the code would be to include the customer's start/end date to the packing slip?

Thank you,

Elizabeth