I have agreed that a customer can pay an invoice by monthly standing order. How do I allocate those payments to their account so that the invoice balance is reduced?

We can use a Receive Payment feature, Roger. To allocate your customer's monthly payment.

Here's how:

1. Click on + New.
2. Navigate to Receive Payment.
3. Select the payment date.
4. Input the reference number and the amount received.
5. From the Deposit  ▼ dropdown, choose a deposit account to record the payments.
6. Select the Payment method used by your customer.
   • Note: If payment is via a credit card and a card reader is available, choose Enter credit card details, then select Swipe card.
7. In the Outstanding Transactions section, choose an open invoice to apply the payment to.
   • Note: To receive a partial payment, enter the amount your customer paid in the PAYMENT field.
8. Once done, click on Save or Save and close.

For additional information on how to accept online payments from your customers, please consult this article: Receive and process payments.

If you need to process a refund for your customer, this article can guide you on how to void or refund payments: Void or refund customer payments in QuickBooks Online.

For any further questions or need clarification regarding receiving payments from your client, please respond to the thread below. We are here and ready to help you.