If the crack in the fiberglass shower is small you can go to Amazon or to a hardware store such as lowes and Homedepot etc and purchase a fiberglass repair kit which is pretty cheap.
The repair kit for the fiberglass tub and showers can be just a fiberglass repair kit which is what it's known as sometimes at hardware stores.
But they may also sell some fiberglass repair kits that are called Tub and Shower repair kits but make sure it's for repairing fiberglass.
Then you use the fiberglass repair kit on the tub or shower to repair the crack and it will last a long time if you do the repair correctly.
The fiberglass repair kit for your shower will come with instructions which will tell you how to actually apply the repair kit to repair the fiberglass shower crack.