To add delivery or other costs chargeable to the customer in DealerKit you can add it as a 'promise' when stacking the deal.
If you didn't add delivery for example when stacking the deal, don't worry. You'll just need to add a job once the deal is stacked by adding a job card.
Any jobs added once a deal is in progress are considered a 'promise'. Call the job 'delivery' and use delivery as the job type. This will add the cost to the invoice for the customer.
You can also add delivery costs as direct expenses to vehicles for vehicle purchase and delivery costs. Direct expenses are specifically designed for costs like delivery.
To add as a direct Expense, you can view the below guide:
https://help.dealerkit.co.uk/en/articles/7965892-vehicles-expenses-adding-direct-expenses