It is quite common to invoice a customer with terms of “30 days or net monthly account”. Small customers will be expected to pay within 30 days, while large customers will be expected to pay at the end of the month following the month of the invoice, so an invoice sent in August would be settled by 30 September 2016. Large companies do it this way because they may receive several invoices from a supplier, and will want to settle all of them with one payment when they do their cheque run. It would be a good idea for the supplier to send a statement at the start of September listing outstanding invoices. Typically the cheque run would be about the 24th to the 26th of the month. If you give credit and have debts to collect, then you might like to have a discussion with accountants such as us.