To add a parking receipt that can be printed for the Customer at a later time, first go to the Settings - Parking Lots tab, then select the parking lot for which you want to design the receipt.
You can use the available template by clicking the envelope labelled templates.
After clicking the purple "show" button, you can preview how the receipt will look in its final version.

Once the receipt has been customised, check "active" and save the action.