Once you send the Invoice to customers, they will receive an email with invoice totals and a link to view the invoice the invoice.
Once they click the View Invoice Button, they will be taken to a screen where they can view all the Invoice details. On this screen, they can click the the link with title 'Pay' to pay one Invoice. On clicking this link, they will see a dialog where they can choose to pay through available payment methods, Paypal or Stripe. At least one of these payment methods needs to be configured by you to enable your customers to pay online. To configure payment methods, you can go to Settings > Invoicing Settings > Payments. Once payment is complete, both customer and you will receive a notification of payment confirmation. In case of online payment, payments will be recorded automatically to the system and customers will receive payment confirmation notifications but if payment was made offline like Bank Transfer or Check or any other method, you need to record payment manually in system as illustrated in next section. on this page, customer can also leave a comment on the Invoice if they like to. You can view the comment on 'View Invoice' page and can reply to those comments too.