By defining services as products, it’s easier for you and your customers to see what they are getting. If you are using the approach to sell service contracts, then your customers will try if possible to look at what they have spent in the past. Generally, as they are paying annually, they will expect a discount compared to ad-hoc hourly rates. For you, the benefit is regular income, less paperwork and possibly less invoicing disputes.
Try as much as possible to avoid adding options to your productised services, it adds confusion and complexity, the very things you are trying to overcome. It also makes customers suspicious, the insurance industry being known for this.