If ‘Display expiry date on order confirmation’ is enabled for the expiry rule at the moment of purchase, this should be displayed in the packing slip automatically.
1. Please make sure WP-Admin > Products > Expiry Rules > expiry rule > ‘Display expiry date on the order confirmation’ is enabled
2. Configure a product so it has an expiry date
3. Add the product to the cart and place an order
4. Check the package slip.
Does it display on the packing slip now?
Does it display in the order confirmation email?