hi there,
you can put any terms you want in your ts and cs, as long as the client agrees them, then all is ok.
it might be worth putting in a clause, stating, that if payment is received by a certian date, then you will suspend the service, however the client will continue to be billed.
also you might want to issue invoices for december service, at the middle of december staing that payment is due by the end of the month of service.
regards
martin