You can add services like delivery, maintenance or advance payment in your product catalog. Services are added the same way as products.To add a new service open All Products or Product Catalog page, and then click on the "+" button or F8.
The difference is when choosing a category. The key is to choose a category that has Track stock option set to No. This means that products added to this category aren't tracked on stock and are treated as services.
Click on CREATE button or press F9 keyboard key to save the changes, or click on the CANCEL button to ignore the changes.