Owner didn't leave enough litter, what should I do?

Considering your concerns, I’d suggest order it online or over the phone for Curbside pick up. Retailers have become really proficient at protecting the community with mask wearing employees bringing items to your vehicle, with receipt (which you can provide for cash reimbursement later).

Personally, I'd be more concerned about the cat's comfort. I'd make an adjustment on the charges to the owner and buy additional litter. But that's just me.