User is emailing invoices to clients, but notices that the option Pay Now isn't showing up. This only happens when there are two or more invoices attached in an email.
Currently, when you email multiple invoices to clients, the Pay Now button isn't displayed. This is by design. Do note that Stripe should be adding the feature to see all outstanding invoices on its website. So, when your clients go to pay the invoices, they can see all other outstanding invoices to pay as well.