Yes, absolutely!
A key component of FlexJobs is the ability to Customize the job 'on the fly' as the changes are needed. You might find there's a Product already included in an existing FlexJob that you'll need to remove just for a particular customer, for instance.
An existing FlexJob can be Customized by removing or decreasing the quantity of any products that are already included. When actually building a proposal, you can still View/Edit any FlexJob and remove the included Products. So, it goes:
A product may be removed entirely from the FlexJob
The quantity of an existing product may be decreased