Why were some of my invoices sent out automatically with reminders when the invoice hadn't been sent yet in the first place?

Let's turn off the automated reminder, coyoteenterprise.

It's possible that the Automatic invoice reminder option was turned on. Let me guide you with these steps on how we can turn off the option:

1. From the Gear icon, select Account and settings.
2. Go to the Sales tab.
3. Click the Reminders section.
4. Toggle the Automatic invoice reminders option to turn it off.
5. Click Save, then Done.

I've also added this reference if you want to let your customers pay the invoice online: Take and Process Payments Online.

Let me know if you need help in sending an online invoice. Have a good day!