I just repaired such a shower using the type of fiberglass and resin product Tom mentioned. but the repair kit was sold at Lowes.
Essentially, you sand the affected area, prepare a fiberglass pathc, mix epoxy resin and hardener, apply a few coats of resin.
You also get colorant to tint the resin to match the existing tub. But this is a very touchy 'guessing' how much colorant to use to get the color match, and unfortunately it is not easy.
That said, the surface repair works but the color was not quite dead on...But something the customer could live with given the $1000 option of replacing the tub.
You should also try to resupport under the pan where is sags down.
Don't know if this is the recommended fix but I drilled a 1/4" hole in the side of the pan near the floor just large enough to fit the spray tube from a can of expanding foam sealer. Then filled the area under the crack so that it was 'supported' by the foam insulation.
The other option is to call the manufcaturer of the shower if you know who made it. They will send a factory trained rep to make the repairs.