After enabling Online Payments, your clients will See a Review and Pay button in the invoice emails you send them. Hitting this button will take your client to where they can choose their payment method. They will also have the option to download a PDF copy on the invoice.
Once payment has been submitted, your client will be directed to a new page indicating that the payment was successful. They will also have the option to download a receipt for the transaction.
After the payment has been submitted, the client will see the updated payment status when they view the invoice.