How to apply late-rent fees-my tenant has past due rent payment

[Yashar Fred]

My tenant has been trying to figure out online payment, her Jun 1 due rent payment was pending for 3 days on the Turbotenant payment portal before it was returned "due to insufficient funds", she had made a second attempt to pay online right away. now the rent is 3 days past due and it will take close to two weeks before her second online payment clears up),

I was wondering should I impose a late fee right away? and what is a good way to do so in this case? the late fee is 10% of the unpaid rent, Can I add 10% for every day the due amount is not paid or is it a one-time fee? 

This is the first time she is late (and for understandable reason, after she found out about the returned payment she told me she prefers to pay with a cashier's check next time, although she had already attempted the second online payment)