per invoice or per total debt

Hi,

I currently have 3 invoices overdue from my agency due to them deciding to close up over xmas with no notice or arrangement to make payments in time.

Each invoice is around £3-4k and the payment terms stipulate payment within 14 days of invoice.

When this last happened I warned them via email that I would charge fees for any future late payments.

My question is, is my company entitled to charge the £70 fee plus interest for each of these late invoices or just £70 + interest for the whole combined debt?

Thanks,

AC