Only generating an invoice is not enough for a busy project manager. We hate to request payments to the clients manually and process them on our own.
To solve this problem, we have made an add-on for WP Project Manager Pro, which works with the invoicing add-on. With the help of this add-on, you can bill the client automatically through the famous Stripe gateway. It is well known for low charges compared to PayPal and other available solutions. And it is specially used to take payments via credit cards.