Q: I hope you can suggest something that will remove a stubborn stain from my fiberglass shower pan. Can you help me? Thank you.
Once scratched, fiberglass becomes harder to clean, as you've clearly found in attempting to get rid of your shower pan's stubborn stain. Your best cleaning bet is a neutral (generally a pH between 7 and 9) cleaner, such as liquid dishwashing detergent, applied with a white, nylon-back scrub sponge. Add hot water and a bit of elbow grease and the stains should be lifted in no time*.
Finally, to prevent future stains from grabbing a toehold, lightly wax the fiberglass with car wax. Really. This sealant will close surface pores that allow stains to set in fiberglass and will make it buff-shine like new. Be sure to use a rubber bath mat so you won't go slip-sliding away in your newly stain-free shower (be sure the mat itself is applied when the surface is dry and adheres firmly to the surface during use).
*Editor's note: You may also want to try a little oxygen-based cleaner (containing hydrogen peroxide) to gently bleach out the stain.