Please my question  is: Do I need to develop  functional requirements  for a whole commercial -off-the-shelf products or do I just  need to do it for the requirements the Commercial -off-the-shelf products does not meet. 

Thanks 

It depends on how far through the process you are.

If you are yet to select a COTS system then you will need functional requirements to understand which system will be the best match to deliver your desired functionality. We often use these requirements to score each system depending on how well they meet the outcomes needed.

Non-functional requirements are needed across both the product selection stage and then once you come to doing the setup and install. Non-functionals will help you select the product required e.g. some may or may not be able to support the number of users you require, some may not work remotely others may, security is usually a hot topic here such as encryption.

So from my point of view, even if you have a product selected and installed you can still define non-functionals to understand how the product should be setup and configured especially when looking at future growth.