Hi,

The most apt way to achieve this would be to create a service product and give units of measure in hours and sell that based on hours. 
That way you can add the product in hours and sell it to your client.
To automatically add a product in the invoice, is not possible by default in odoo, you need to customise.

Hope it helps

Hi David,

It will be done by some customization in which you will have to link your service product with the product you are selling. Whenever you add your main product to the invoice, other linked products will also be added to the invoice.