Clients, packages and invoices live in the same dashboard as your servers - this is white label hosting done properly: add a client, put them on a plan, and the billing side follows without a separate tool.
Each client is a real account under your agency - not a shared login. Put them on one or more packages, and the panel places their sites on whichever of your servers has room. When a client leaves, removing them tears down their sites and databases properly instead of leaving orphaned files behind, while keeping their billing history intact.
A package is more than a price tag. You set how many sites it allows, how much storage and database space, whether staging and email are included, and how backups work - then decide the price, an optional discount with its own expiry, a trial period, and a grace period before a lapsed plan is cut off.
Run invoices for a period across every active client plan, then mark them paid as payments come in.
Know who's behind without digging through a spreadsheet.
See what each client is paying against what each server actually costs you - and the profit in between.
A support ticket inbox and live chat queue sit alongside everything else, so questions about a client's site don't need a separate helpdesk tool. Add support staff of your own once you're past handling it all yourself.