" I was posting on CL with low rates "....
Sounds normal for a craigslist deal to be honest. Techs on there are usually there for a reason and legit businesses who shop for techs on craigslist...? Doesn't usually add up.
I'd raise your prices closer to 'normal' rates. Based on your experience it sounds like you really don't belong on CL with those guys. If the company wanted to pull a fast one they would most likely hunt down a 'cheap' CL tech as they would think they weren't legit or they wouldn't have the means to pursue them anyway. There was a good article somewhere relating to that called reverse scams or scamming the scammers or something like that.
Hopefully your gut feeling and them being an engineering company is right and you'll wind up getting paid. The only concerning part is why would a legit engineering firm use craigslist to get tech work done. Usually they call a reputable firm, unless they have a bad rep or an issue with paying.
I would do more word of mouth and other advertising. Your experience doesn't really fit in with the normal CL tech crowd to be honest.