Priming or painting isn't needed before applying the gel stain to an embossed (wood grained) fiberglass door. All that is needed is a clean sound surface. Plus your door should already be a light tan color, perfect for staining over.
Depending on the door, and if it has sidelights, $500 isn't a bad price.
There shouldn't be any problems waiting until spring to stain the door(s). Just make sure to wash the door surfaces with a mild household cleaner and allow to dry before applying the gel stain.