You can use a discount SKU (Create a SKU code (virtual/service) and give it an appropriate name: (eg: 99_001, 99-002 etc., here we assume all "service" products will have a 99 prefix). You may need to add it under a "services" product category for easy reference in reports.

Then, you can include that SKU and manually add a negative price at the row (last row) of the invoice for that SKU.