I ran into this issue as well. Now I issue receipts (invoices) 2 days before payment is due specifying the amount so there is NO confusion as to what they owe. And, as others mentioned, have your contract handy to show and invite her to read it over. It could actually be that she's forgotten. When my son was in daycare, many moons ago, I shorted a paycheck by a day as he was ill. The daycare I had dealt with years before for my daughter allowed sick days, so I just assumed it was standard protocol. They pointed it out in the contract and I paid it up without a fuss. Is there a chance it could be as simple as that....actual forgetfulness? At any rate, if you issue invoices, then clients aren't guessing what they owe....it's on paper.