I just generated an invoice and said to be paid within 30 days but it shows as paid already when it hasn't.

Welcome to the QuickBooks Community, @m-amier88.

Making sure you have an accurate record of your sales transactions in QuickBooks Online (QBO) is my priority. 

To help you with this, let's first check your settings and make sure that the Automatically apply credits option is turned off. This may be the reason the invoice was marked as paid even no action was taken. Here's how:

1. Go to the Gear icon and select Accounts and Settings.
2. Click Advanced.
3. Navigate to the Automation section and make sure that the Automatically apply credits option is off.

On the other hand, you can also check your Audit Log. This will show you the history of any activities made to the invoice. Let me guide you how:

1. Click the Gear icon and choose Audit Log.
2. To filter the data, select the Filter drop-down.
3. Select the date range.
4. From the Events field, click Show only these events and check Transactions.
5. Once done, select Apply. 

In case you want to have a list of your customers’ outstanding balances, you can run the Accounts Receivable ageing summary report. This report will show you how much are still due and how long they're past due.  

If there's anything else I can do to further help you in managing your invoices, please post a response below. I'm always available to help in case you need it. Have a good one.