The above invoice for ${enter amount} remains unpaid despite our previous reminder. As such, we would appreciate you making this payment as soon as possible.
I value your business but regret that we are unable to continue supplying goods or services on credit while this amount remains unpaid.
If there was a problem with the goods or services supplied I urge you to contact me so that the matter can be addressed.
Otherwise, if there is a problem paying this amount owing I’m happy to discuss an alternative arrangement for payment so that we can get your account back onto normal supply as soon as possible