Once you have a credit card integration set up through Stripe or WorldPay, your customers can make their payments directly to you through an online payment portal.
You can include a link taking them directly to this portal by using the 'invoice_portal_url' tag when setting up the 'Invoice email message' from within the 'Invoice' section of your system settings.
If you do not see this option, please check your 'Payment methods' to ensure that you have enabled the setting 'Use credit card integration', for at least one of your payment methods.
Once this has been selected, the tag option for the invoice portal URL will be available to be utilised to display the link in the emails that will be sent out regarding invoicing.