We do a whole lot of work for national companies like this. In fact it's my main line of work. As noted above you MUST check out the reputation of the company at the syndicate. There are a few to avoid like the plague and until you've done this for a long time that other site is a godsend. The list is too long of deadbeats to go into here. Also, net 30 or 45 is normal anymore but you can often work out a deal like 2% discount if paid in ten days etc. Tell the ones who insist on net 90 or 120 to take a hike.
Make sure to outline everything in your quote. Permit fees, licenses (if you need to pay a local license "tax" instead of a state license), engineering, etc. are all usually billed at cost. Don't forget to price out the permitting at an hourly rate instead of a flat fee. The permit might be an over the counter 15 min affair or require several trips to the township, sealed plans, electrical seal, etc. Permitting can cost more than the job in some cases. The survey fee is always an extra, though some will only pay XXX dollars for a survey. Oh, and the ones who ask you to install without a permit, tell them to walk as well. You are the one responsible, not them no matter what kind of "waiver" they throw at you.
On change orders get it in writing BEFORE doing the work. Take extensive photos of the work when done. Get a sign-off sheet from the GC or manager onsite and insist they print their name next to their scribble. And know how to file a lien on the property if it comes to it.