4. O ile przy dokonywaniu płatności nie zostanie wskazana wprost faktura, której płatność dotyczy, zapłata zostanie zaliczona na należności wynikające z najdalej wymaganych faktur.
5. W przypadku braku zapłaty należności w terminie, Sprzedający naliczy odsetki maksymalne w stosunku roku od dnia następnego po dniu wymagalności do dnia zapłaty. - http://www.goldfinchtrade.com/warunki-handlowe/

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4. If at payment no invoice that the payment concerns is provided, the payment will be applied towards liabilities resulting from the invoice due that was issued the earliest.
5. In case of lack of timely payment, the Seller will accrue maximum interest on annual basis from the day following the due day till the day of payment. - http://www.goldfinchtrade.com/en/general-trading-conditions/

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4. Payments made by the Buyer shall always initially be used to pay all interest due and costs and secondly the earliest invoices due even if the Buyer states that payment relates to later invoices. - http://tricover.com/wp-content/uploads/PDF10_Voorwaarden_Hey...

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Payments of trade receivables are allocated to the specific invoices identified on the customer's remittance advice or, if unspecified, are applied to the earliest unpaid invoices. - https://ishr.site/FgYo

This makes it abundantly clear that we are going back in time.

During the month end process, any payments are allocated to invoices on the basis of the oldest outstanding invoices being allocated first.
http://alphalogix.co.uk/wp-content/uploads/2018/01/Sage-200-...

Overdue payments that You make to Us will first be allocated against any late charges arising and then the balance allocated against the oldest outstanding invoices.
https://towcesterselfstorage.co.uk/homepage/towcester-self-s...

Payments received will be allocated against the oldest outstanding invoices first.
http://www.servicecentre.co.uk/reseller/

Please note that we will allocate your payments to the oldest outstanding invoices.
https://www.bonnersmusicschool.co.uk/terms-and-conditions