Odd shops, small firms ( no more than a couple of hours work at most ) are pretty safe payers. Good to have a few of them. Depends how big you want to get. I had the British Army's contract for all toplights on the barracks here in the 90's. Over here they set prices by square mtrs. The Army measured it themselves and made the mistake of counting both sides of the glass. Gave a normal price and they accepted, DOUBLE MONEY lol!!!
With bigger companies, they usually go with the cheapest IMO. Loyalty doesn't seem to come into it. Lost a batch of Schools because a firm quoted 3 cents cheaper than we were charging before.
As for paying, Army and local councils here pay in theory after 90 days, more like after 120 days though in reality. Shops and small businesses vary. Some chain stores wanted an invoice after 90 days, then took upto 120 to pay.
It's good posture over here to give a 30 day to pay invoice though. In the end I look at it like this;
If I have a few hundred domestic custies, it would be unusual to lose more than 1 or 2 a month. If I have 20 big commercial jobs and lost only 1 of them, I would feel that at once. Big numbers, big mistakes, small numbers, small mistakes. I prefer less stress and safety, but that's just me.
Hope this helps.