Many small pieces, many clients, and a lot of admin per unit of revenue. Keep the overhead proportional to the work.
Save a line-item set once and reuse it, so a new invoice is a few clicks.
Overdue invoices trigger a payment reminder without you deciding to send it.
Every invoice gets a public page where the client can pay by card immediately.
Every commitment on one calendar, with your next free date derived from it.
Free forever for 2 projects and 2 clients. No credit card, no trial clock.