I don't get what people don't understand about how monthly bills work.
"But I already paid you once!"
"Yeah, for the work we did two months ago. This is the bill for the work we did last month."
"... But I already paid you once!"
"Yes, and then you asked us to do more work"
"... ... But I already paid you once! Why are you trying to rip me off!?!"
*sigh*