Hello, Charlie. I have some information about why your estimate changed to an invoice.

In QuickBooks Online (QBO), once your customer accepts your estimate, you have the option to convert it to an invoice. But, if you have QBO Payments and you're using the new layout, an invoice is automatically created after your customer pays the deposit. If so, you can refer to this article for more information:  Convert an estimate into an invoice in QuickBooks Online. 

However, if none of these applies to your concern and you're referring to something else, I'd like to ask for further details or a screenshot so that I can provide an accurate resolution. 

In addition, I've added an article for your reference to guide you on how to receive and record invoice payments efficiently: Record invoice payments in QuickBooks Online.

I'm always here to help you if you have further questions or concerns about your estimates and invoices in QBO. Looking forward to hearing from you.