What about APEX?
In my opinion and experience, the market does not like APEX certificates.
If you are getting something expertized for your own collection and have no plan to dispose of the item within the foreseeable future, then APEX is certainly viable and the cheapest mainstream expertization option out there. I have several items that I got APEX certed as part of a StampStore purchase; for that it is a great bargain.
If, however, you plan to sell, I would avoid anything not PF or PSE (ideally the former) for U.S. material.