I would strongly recommend you do NOT order that paint kit sight unseen. I work for a major coatings company in the automotive refinish division. White is an extremely difficult color to match for vehicles in general and impossible to do with a pre-mixed kit off the internet. You would probably end up with shiny fenders in the end, but I can all but guarantee that it will be different enough from the rest of your trailer that you will notice it (it is amazing how a subtle difference in white can make such a huge impact when two panels are compared). The kit you are looking at is meant to be sprayed out of an automotive refinish spray gun. How are you planning on applying the paint? The kit also has a hardener with it which means that you have a limited amount of time to be able to spray the paint correctly before it begins to set up and solidify.
I would recommend visiting an automotive refinish store (also called Jobber store) and talking to someone inside. They will be very knowledgeable about how to prep correctly for adhesion and gloss and they will be able to give you an accurate match if you can bring one of the fenders to them. Fiberglass is not much different to paint on top of than bumpers on cars or other parts. Lots of people out there are building fiberglass hot rods and have been repairing Corvettes since 1953 so fiberglass trailer fenders are not out of the ordinary for an automotive paint technician or jobber store clerk to handle. I would also bet that someone inside a jobber store near you does paint work on the side in their house and they could do a great job for you if you don't have the correct equipment or skills.
I would be glad to offer step by step advice privately if you would like to tackle this yourself - just shoot me a PM.