Issue with invoice. Invoicing an existing customer and QuickBooks keeps saying the invoice has been paid before I’ve sent it.. meaning the invoice sent is for £0.00.

Greetings, @info-admk-electr. 

I'm here to clarify things about invoicing in QuickBooks Online. 

When the invoice amount is zero, it will automatically be marked as paid. If you're trying to save invoices with a value greater than zero, it shouldn't show up as Paid. Unless there is an existing credit that is automatically applied to the open invoice.

To avoid getting the message, make sure that the invoice amount is not zero. You can also check the email history of an invoice. By doing so, you can see every single change and action performed to the transaction.

1. From QuickBooks Online, navigate to the Sales tab and select the Customers section.
2. Choose the name of the client whose invoice you'd like to know the email history of.
3. Find their invoice from the list below. If necessary, use the Filter menu to narrow the transactions displayed. 
4. Click the invoice to open it.
5. Click More at the bottom and select Audit History.

Also, you'll want to learn how to manage customer payments in QuickBooks Online. Here's an article you can browse for more details: Get paid by your customer.

I got you covered if you have any other concerns with sales transactions in QuickBooks. I'm always available to help. Have a lovely day!