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 September 2018 would be settled by 31 October 2018.
Large companies do it this way because they may receive several invoices from a supplier, and will want to settle all of them with a single payment when they do their computerised cheque run. It would therefore be a good idea for the supplier to have sent a statement at the start of October listing all outstanding invoices. Typically the cheque run would be about the 25th of the month, which is the date of this posting. If you give credit and have debts to collect, then you might like to have a discussion with us. Most accountants are also general business advisors as well.
The “net” in “net monthly account” has “nett” as an alternative spelling.