Usually, when selling both goods and services, you are bound to sell them together within one package at some point. Normally, provided you sell both separately, you would have a price for each of them and recognizing them in revenue isn’t at least an issue when it comes to determining the sum to be recognized.  

However, say that you normally sell the goods and not related services; you wouldn’t have a determined price for the service itself. I think it’s easier to explain this through an example – say you’re selling a machine with 2 years of maintenance service being provided in addition. The selling price is 10,000 altogether. Now for you, it’s two types of revenue – for the machine and for the maintenance service. Let’s pretend that the expected cost for the maintenance is around 500 and that the average industry gross margin for such services is 20%, the gross worth would be 625 (500 / 80 * 100). This gross worth is now spread into revenues over this 2 years period.

In a nutshell, even though you’re selling them together with one determined price (be it even the price of the item itself and it may seem like the service is provided for free), you still ought to show revenue from the sale of the service just as well.