How do I balance an invoice when the customer/payee deducts a 10% handling fee? E.g. inv £780 payment £702

Welcome to the Community, Ian. 

In QuickBooks Online (QBO), the option to enter a percentage rate for handling fees is unavailable. However, you can calculate it manually and enter it as a flat amount in invoices. I'm here to guide you on how. 

First, create a service fee item. To do that:

1. Go to the Gear icon and select Products and Services.
2. Click New and select Service.
3. Enter Handling fee in the Name field.
4. From the Category dropdown, select the category that best describes the fee. 
5. In the Sales price/rate section, enter the 10% of 780, which is 78.
6. Fill out other necessary information and select Save and Close.

Next, add the service fee to the invoices. If you've already sent them, create another one with the service fee. Otherwise, include the handling fee before sending it. You can refer to the steps below.

1. Go to + New and select Invoice.
2. Select a customer in the Customer dropdown.
3. Select the product under the Product/Service column in the first line.
4. In the second line on the Product/Service column, select Handling fee and enter it as a negative amount.
5. Select Save and Close.

For more details, please refer to this article: Manually add service fees to invoices in QuickBooks Online.

When the customer pays the invoice, you'll have to record the payment in QBO. Doing so connects the payment transaction to the original invoice, ensuring your accounts remain balanced. 

Please know I'm only a couple of clicks away if you have questions concerning invoices. I'm always available to answer them. Keep safe!