There is no requirement they send a monthly statement. When you contracted for the unit you agreed to pay, and absent a writing stating they will send written invoices, your duty to pay still exists.
In these cases I recommend you print out your own "coupons" in advance so you can remember to pay the bill on time.
PLEASE NOTE: The responses made here are for information or education use only, and are NOT legal advice and do not form an attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.