Assuming that you are going to a shop to have your transmission flushed, you may want to check with them first since many shops are contractually obligated to use a specific brand of oil and fluids, and they are not likely to discount their price because you bring your own fluid. Yes, it's a rip off and you should go somewhere else if that is the case.
I have used a local Kwik-Kar Lube to flush my transmissions and they use a 100 percent synthetic Mercon-V made by Pennzoil. Royal Purple is very good, but it is expensive, and can be difficult to locate in some locations. Remember that the Sport Trac transmission will require 14 quarts of fluid for a proper flush. I prefer a fully synthetic fluid like Pennzoil's or Royal Purple. Ford's Mercon-V is a semi-synthetic which is fine, but synthetics tolerate more heat. Heat and damaged fluid are the enemy of automatic transmissions.
...Rich