How do i send an email with a link to existing clients asking them to sign up for autopay?

Hello there, realpurdy1, 

The supported payment gateway in QuickBooks Online is QB Payments. If you're already signed up for that, your clients don't have to sign up when paying their invoices online.

They can simply click the payment link attached to the invoice and pay.

Here's an article to learn more about QB Payments: Connect your GoPayment account to QuickBooks Online.

I'll be right here if you have other questions.