Yeah, I eventually figured it out. In short, I created a field in the invoice called Quickbooks_Invoice_URL that holds the URL. I then make a call out after the invoice is created where the parameter include=invoiceLink is in the URL. That will send you back a value under the invoice object called invoiceLink. Then add that field into your invoice template as an action and anytime any clicks your button in your template, they are taken to that link to pay the invoice.
"https://quickbooks.api.intuit.com/v3/company/{qboInvoiceId}/invoice/{qboInvoiceId}?minorversion=65&i..."
Reach out to me if you get stuck, happy to help you continue to figure it out.
