Welcome to the Community, Ronald! It's nice to see you make your first post! Managing your invoices is an essential step in keeping track of your sales transactions effectively. I'm here to assist you in understanding why they are automatically marked as paid after you save them in QuickBooks Online (QBO).
This happens when the Automatically apply credits feature is enabled. QuickBooks automatically apply credits to your customer's open invoices. If you want to decide which one you want to apply credit memos to, turn this option off by following these steps:
- Go to Settings and select Account and settings.
- Choose the Advanced tab.
- Click Edit in the Automation section.
- Turn off Automatically apply credits to apply credit memos manually.
- Hit Save, then Done.

Right after, you can create a credit memo and apply it to your customer's invoice manually (if necessary). For the comprehensive guide about the steps above, please see this article: Create and apply credit memos or delayed credits in QuickBooks Online.
Moreover, would you like to elevate your customer interactions by adding personalized details to your sales forms? If that sounds appealing, I have some fantastic resources for you: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Additionally, have you had the opportunity to reach out to our QuickBooks Live Expert Assisted team yet? They specialize in simplifying revenue and financial management just for you. I encourage you to explore their services, as their expertise could be a valuable resource for all your accounting needs!
It's my pleasure assisting you, Ronald! I'm looking forward to your return to the Community whenever you need support with your sales and income transactions. Feel free to reach out anytime. Wishing you a wonderful day ahead!