I have done in the past and the water was stone cold and the heat pack not activated properly, although this would have been down to the company not the courier. However all fish were fine except an African Dwarf Frog which was DOA. I think courier is much safer to use than sending fish with royalmail, however mailing usually costs about £7 whereas courier costs about £15. I would use it again if I was ordering a hard-to-obtain fish, or many fish.
So basically the issues I see are the price (which I understand probably cant be lowered) and the welfare of the fish. Perhaps if you offered a service where you checked halfway on the journey whether they were warm and coping alright?
If courier service was offered instead of/as well as royal mail on the ebay auctions etc where you can buy unusual or expensive fish, e.g show bettas, I'm sure more people would take it, however it is not offered. Perhaps if you got in contact with some of the private breeders and importers they would offer courier service as well.