How can we improve WHMCS?

I think it would be a great feature if the following were to be implemented:

Interest automatically calculated on overdue invoices at the end of every month based on predefined settings.

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[u]Example[/u]

Assuming interest is 4%, and is applicable after 60 days.

Invoice of £1000 is due January 31st 2017. Reminders are sent as defined in WHMCS settings. When 60 days have passed, the date will be April 1st 2017, and so on the 30th April (end of the month), the system calculates and applies the interest to that invoice, which in this case would be £13.20.

The calculation based on the assumptions of this example are

£1000 x 0.04 (4%) = £40

£40 / 365 (days in year) = 11p per day

11p x 120 days overdue = £13.20

If the invoice is then not paid by the end of May, the system should calculate, on 31st May, the interest based on the original amount overdue, now for 151 days, and so the calculation would be:

£1000 x 0.04 (4%) = £40

£40 / 365 (days in year) = 11p per day

11p x 151 days overdue = £16.61

Bare in mind it would have already added £13.20 of this to the invoice, and so would need to add £3.41 to make up the interest for May.

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To accompany the above, it would be nice to see a button within the invoice view that calculates and adds the interest up until the current date so that if we need an up to date outstanding amount in the middle of the month, we can get it.