I've considered having a flash cart printed before, just never bothered to look fully into it. My main concern with this right now is that I know for timing most SNES boards are CF, due to the speed, I wasn't aware that they had SD NES boards, and I wonder how they'd compare.
I wouldn't make them to sell, I would make them if someone asked for one.
The SD carts have been around for several years. It's just you'll need an top loading NES or clone. I don't think they went into making the CIC chip for it.
But the one I wish I could track down was the PS1 one where you can load anything and it'll run it. But I think you were removing the laser and hooking the SD/USB card into the system.
I just found it odd that the NES could take SD when all of the SNES carts I've seen use CF and for a reason, was just odd in my head.
But yeah, as long as you're breaking even, and it's not public, don't see any issue. You could always call up the place and see if they have any requirements on the specs you send, limitations, etc.
I haven't looked up the different prices, but I just know that the person I know who has used their services has been in electronics for decades, and enjoyed the service.