Dear :

I am sorry to inform you that, despite all our efforts, we won't be able to clear your [month] statement for $ by [date]. [reason].

Could you extend the payment deadline till [date]? Our insurance company has promised us compensation within the next few weeks. As soon as we receive this, we will pay the account in full.

We would be most grateful if you could understand our situation.

Sincerely yours,

[name]

[title]