Yes, you can accept only one payment method for invoices in QuickBooks Online (QBO), @asmock. I'll gladly share details about this below and guide you on how.

Regardless if you don't or have a credit card merchant account with QuickBooks, you can choose your preferred online payment. To do this, you'll have to manually uncheck the Cards option in the Online payments section before sending your invoices. Please see the screenshot below for your reference.

You can refer to this article if you wish to learn more about accepting electronic customer payments for online invoices in QBO: Take and process payments in QuickBooks Online with QuickBooks Payments. 

Also, you may want to check out this article as your reference to guide you on how to check when QuickBooks deposits customer payments into your bank account: Find out when QuickBooks Payments deposits customer payments.

Please feel free to leave a comment below if you have other online payment concerns or questions about managing invoices and sales transactions in QBO. I'm always ready to help. Take care, @asmock.