There are zero upfront no-questions-asked 3D printing services as well as such PCB/PCBA manufacturing services, but not a Buy Now style Mechanical Turk assembly

There are zero upfront no-questions-asked 3D printing services as well as such PCB/PCBA manufacturing services, but not a Buy Now style Mechanical Turk assembly line service, yet. So you'll have to do the non-scalable work and do the literal manu-facturing when it comes to tiny volume cottage industry products with no cash or volume to foot and amortize upfront costs. Which can be bypassed or minimized if it's supposed to be a kit and you only have to bag some parts. Also it blurs areas of responsibility in favor of the shop.