A prequalified WPS is simply one that meets all the prequalified criteria specified by D1.1 Section 3, thus the procedure does not have to be qualified, i.e., there is no PQR required.
Anyone can write a prequalified WPS.
The issue you point out is one of ethics. Is it proper for a third party inspector, being paid by a client, to inspect the work performed by a contractor and "sell" WPSs or charge for any other work or services to for the contractor during the course of the project? That is, should the inspector or inspection agency collect fees from both the contractor and the client on the same project?
My reply is "no". Other people my disagree, but this is my opinion.
I get around that problem by charging my client for all the work done on the project. If I test the contractor's welders, my client pays for the testing. If I have to write a WPS for the contractor, my client pays my fees. My client can absorb the cost or pass it on to the contractor, but in any case I am not collecting money from the contractor and my client on the same project.
As a third party inspector, I will not provide the contractor advice on "how to do something" for a project where I am being paid by the client. If the client asks me to provide an opinion or some direction on the solution to a problem, I do so, but still, it is the client that pays for my services, not the contractor. I usually tell the contractor that they should retain the services of a different "welding consultant" so that I maintain an arm's length from the contractor doing the work to maintain my objectivity and to minimize the appearance of any conflict of interest.
I just finished qualifying a welding procedure for welding carbon steel to stainless steel with a contractor that will be performing the field welding. My client is the owner on this project. The contractor provided the materials and labor, but my fees for witnessing, recording data, testing, and writing the PQR and WPS were paid for by the owner, my client. We will be qualifying the welders next. Again, the contractor will not be charged for the costs, my client will pay my fees. Whereas my fees are paid by my client, the welding documentation is very specific to the requirements of this project. This arrangement avoids any conflict of interest that would arise if I collected money from both the owner and the contractor.
I hope this explanation and example helps you to understand your situation.
Best regards - Al