Where does the payment instructions appear on the invoice?

The Payment instructions will appear right after the Balance due of the previewed invoice, frossini.

The previewed page is where you are about to send the invoice to your customer. The screenshot below is a sample invoice I've created. I've set "follow these steps" as the Payment instructions.

Meanwhile, you customer will see it on their email preview. When your customer click the Review and pay button, they will be routed to a new page to see the invoice separately from the email. The Payment instructions section is at the bottom of the page after the credit card information. I can't provide a screenshot for the customer's view due to restrictions.

When the customer successfully sent the payment, you'll receive it within 5 business days if this is your first time to receive an electronic payment. It will take a bit longer because we are setting up your account. The next payments will be processed much faster. You can check this links to know more about the process: Find out when QuickBooks Payments deposits customer payments.
 

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From there, you can already record the invoice payment and create a bank deposit. Furthermore, you can categorize those payments if you setup the Online Banking and reconcile your account with ease.

We'll be right here if you have additional questions with processing electronic invoices for your customers. Have a great day!