When I create an invoice for a particular customer and save it, it then automatically puts it as paid, even though it hasn't been. How to I fix this?

It's nice to see you here, @millysmuddymutts. I'm here to help you with your invoices.
 

In QuickBooks, invoices won't automatically be paid or applied for the existing credits when the Automatically apply credits feature was turned off from the Account and Settings. 

Before that, clear the payment of the said invoice and ensure that this feature is off from your settings:

1. Go to the Settings (Gear icon) menu.
2. Select Accounts and Settings.
3. Click the Payments tab, then go to the Automation section. 
4. Uncheck the box for Automatically apply credits.
5. Click Save, then Done. 

In case this setting is already turned off and the invoice was marked as paid, It could be that the invoice was accidentally marked as paid. We can check your audit log to confirm. I'll show you how: 

1. Click the Gear icon, then select Audit Log.
2. Filter the transactions from the Filter drop-down arrow, then click Apply.
3. Locate the invoice and see who received it.

You can also check these articles on how to send and customize invoice emails:

• Send an invoice
• Customize your invoice emails
   

If there's anything else you need, you can always post your concerns in the Community. I'll be around if you need my help in emailing and managing sales transactions.