If you have a concept for a design, here is my experience. I used http://www.protomine.com to develop my technology myself. Had a designer locally draw up the drawings and submit them directly online. It was pretty fast and cool. They made me a 3d graphics package and a prototype in about 7 days for 10x less it would cost to develop my invention through other invention development channels. I also had them submit the presentation directly to some companies which was cool. I would avoid mainstream “invention submission companies” as they are pretty slimy. Goto the United States Patent Office site to see all the complaints and lawsuits (type in “invention complaints” in Google and goto the Patent Office website). Also avoid rushing into getting a patent as it is vary rare that someone steals an unproven concept. There are millions of “concepts” out there. The cost to actually develop a product from scratch including market research just to make “sure” it will be profitable is roughly $100k. Most inventions are stolen after “proof of concept” which means they’ve been tested to work. It would be like betting on a horse that’s never raced vs. a sure thing.
Anyway best of luck on your invention design development.