A typical professional services engagement is never just one thing. You bill a retainer base ($3,000/month), deliver fixed-scope project work (phase 1: $8,500), and then track hourly overages ($150/hour) when the client asks for extras. The question: which invoicing platform handles that on a single bill without forcing three separate invoices, manual line-item juggling, or lost time tracking? We ran a real three-month scenario across Wave, Xero, FreshBooks, and Orin to see which platforms auto-prorate recurring retainers across billing cycles, which ones lock you into one billing model, and which ones can actually switch your model mid-engagement without breaking the math. The test: Three months, one messy client Meet Client A. Month one: you invoice a $3,000 retainer (recurring) plus a $5,000 deposit for Project Phase 1. Month two: Project Phase 1 completes ($5,000 fixed); you bill another $3,000 retainer plus $900 in hourly overages (6 hours at $150/hour). Month three: the retainer continues, no project work, but $2,100 in overages (14 hours). Total across the three months: $17,400 in revenue, split across three different billing models on the same account. The problem: most platforms are built around either recurring billing (subscriptions) or project billing or time tracking. You pick one and live with it. When you need all three on one invoice to one client, the workflow breaks. Wave: Free but inflexible Wave's strength is price (zero) and simplicity. Its invoicing UI is clean, and it integrates with Stripe and PayPal for payment collection. For a freelancer sending one type of invoice repeatedly, it works. Here's where it breaks: Wave has no native recurring invoice automation. You can create a template and copy it manually each month, but there's no scheduled invoice generation. For Project Phase 1, Wave is file-based—you estimate a fixed amount upfront, create one invoice, and mark it done. To add retainer + project + overages on the same bill, you'd manually line-item each component into a single invoice. That works once. By month two, when you want to auto-bill the retainer and add new overages, you're back to copying templates and editing line items by hand. Wave also has no time-tracking module. If you're working hourly, you log time in a separate system (Harvest, Clockify, Toggle) and manually enter hours into Wave's invoices each month. For a three-person team billing the same client differently, this creates reconciliation risk—hours logged in one tool, invoiced in another, with no link between them. Wave is free and good for single-stream billing (retainers only, or projects only). For mixed models, the manual overhead outweighs the savings. Xero: Powerful but requires setup discipline Xero handles all three billing modes—recurring invoices, project tracking, and time sheets—but they live in separate modules. A recurring invoice bills your $3,000 retainer automatically on the 1st of each month. Projects track the $5,000 Phase 1 work with optional time entry. Time sheets log hourly work. The integration between them is manual. To create month two's invoice, you'd need to: Let the recurring invoice auto-bill the $3,000 retainer (automatic) Manually pull time-sheet hours from the time-sheet module and add them as a line item ($900) For Project Phase 1: either bill the fixed $5,000 from the project (if still open) or add it manually (if closed) Xero does not natively consolidate these into one invoice. You either generate three separate invoices and manually combine them into one PDF (clunky), or you create one invoice and manually add line items from each source. By month three, when you want to switch Project Phase 1 off and keep retainer + overages, you're managing that toggle in three different places—recurring invoice settings, project status, and time-sheet assignment. Xero also does not prorate a recurring invoice if you change the amount mid-cycle or stop it mid-month. If you wanted to reduce the retainer from $3,000 to $2,000 partway through month two, Xero would bill the full $3,000 again and you'd need to manually issue a credit note and adjust the next invoice. That error risk compounds across clients. FreshBooks: Blended billing, but not seamless FreshBooks was built for service professionals and has the most integrated approach to mixed billing. It combines recurring invoice templates, project costing, and time tracking in one product. Here's how it handles your scenario: recurring invoices auto-bill the $3,000 retainer. Projects can be attached to a client and tracked hourly. At the end of each month, you can generate a project invoice that includes both fixed project fees and time-tracked overages on one bill. The workflow: Month one, you create a recurring invoice (retainer) and a project invoice (Phase 1 deposit). Month two, the recurring invoice auto-generates; you add time entries to the project; FreshBooks can combine the retainer, project fees, and time overages into a sin