First of all, sign up with XenFax by providing the necessary details. After successful registration, login to your XenFax account and add your credit card in the Cards section under Account Settings.
Once you have added the card, click on Buy Fax option under the icon. Provide your Area Code and click on the Search button.
Choose your fax number from the available numbers and click on Add to Cart. Now click on Checkout form the Cart section and complete your purchase.
You can refer to this article to know the buying process in detail.