Can be touchy. My former boss got burned on a night call to a GC badly needing a Panerra Bread location finished. Same issue with w9 forms, etc. Plus, their HO was in Texas, we were in NC. I'm sure we would have to look up to see the bottom of their list. They quit taking our calls. He nearly got burned by an apt complex too. I chased and called them constantly, finally got our $. Found out they had tried to burn every contractor in 2 cities.
Ask around about companies best you can and be sure they are solvent. If you smell a rat, ask for COD on 1st job. 1st time late night calls, ask for COD in advance. Lots of shysters out there. If you have to do 30 day stuff, reflect it in your billing. Print on invoice a discount for early payment, etc. If a place calls for service when a bill is lingering, that's a perfect time to mention it.
Nothing wrong with being flexible, but more & more people want to give us last priority.