In the invention process, you have a multitude of choices and opportunities. It is ultimately your decision when it comes to manufacturing as you are the customer and can do as you see fit. If you choose or feel that you can do a better job in assembling and packaging your product, you have the ability to do so. Have the individual parts shipped to your location to begin the assembly process.
Assembling your own product can be time consuming. Not only do you lack the resources that large scale manufacturers have; you also lack specific knowledge of manufacturing components and practices. Although it is cost-effective, self-assembly is usually a small operation in comparison to the vast assembly lines of manufacturing plans. All in all, despite saving a dollar, it can spend precious time, which could still cost you in the long run.
Let manufacturers assemble your product because, as I mentioned before, it costs a fraction of the price of your product and frees up your time for vital tasks such as product marketing and sales. However, if you are looking for peace of mind, you always have the opportunity to visit the manufacturer and arrange quality control procedures to ensure the products meet your needs and specifications.
The truth is that you will always feel that you can do a better job; however, once you finalize your quality control specifications, there is no excuse for the manufacturer to produce less the quality parts. If they do, then you have every right to refuse the product and request the quality specified.