Attendance at the site is the whole contract. Check-in on the premises, with time and person, is what lets you answer a complaint with a fact instead of an apology.
Supplies are consumed at the site and paid for centrally. Tracking what each site uses turns a vague cost into a number per contract, and that number is where the margin actually lives.
Cover for absence decides the month. A list of who can reach which site, kept per person, means a replacement is a phone call rather than a lost morning.