We are currently using WHMCS for billing if you plan to have recurring charges etc. It offers a lot of features and is relatively easy to use. You can get more information on their website: https://www.whmcs.com/ or you can order it through GoDaddy if you use their hosting and/or domain services.
We’ve also tested another one called Zoho which is FREE for up to $50K in revenues. It also has additional modules like a CRM, Help Desk system etc. User this link to find out more: https://www.zoho.com/us/subscriptions/pricing/