You sign a customer to a $2,000/month retainer on the 15th. On day 22, they want to add a service tier worth $800/month. Your invoicing tool either handles the proration cleanly—crediting the overage, splitting the remaining month fairly—or it forces you into a spreadsheet to recalculate, reissue, and track partial credits manually. Most tools do the latter. We tested how FreshBooks, Wave, and Stripe Billing handle three real scenarios: mid-month upgrades, usage overage billing, and partial refunds. The gaps matter because one bad mid-cycle change can cascade into misaligned accounting, confused customers, and hours of reconciliation work. The three scenarios that break most billing tools Before comparing tools, let's define what "working" actually means. Three situations expose weak proration logic: 1. Mid-month plan upgrade Customer on a $2,000/month plan starting the 15th. On day 22 (7 days in), they upgrade to a $2,800/month plan. The billing tool must: Calculate the daily rate of the original plan: $2,000 ÷ 30 = $66.67/day Calculate days remaining in the cycle: 9 days at $66.67 = $600 Calculate the new plan's daily rate: $2,800 ÷ 30 = $93.33/day Calculate those same 9 days at the new rate: $840 Charge the difference: $840 − $600 = $240 (plus tax, if applicable) Issue a single, coherent invoice line—not three separate charges 2. Usage overage on a capped plan Customer on a $1,500/month plan with 10,000 API calls included. Month closes with 14,500 calls used. The tool must: Meter the overages from your platform or usage API Prorate the overages to a per-unit price (e.g., $0.05 per extra call) Roll the overage into the same invoice cycle without forcing a separate charge or invoice Update the next cycle's baseline if the customer's usage has shifted 3. Partial refund and future credit Customer pays $2,000 on the 1st. By the 12th, they downgrade mid-cycle. The tool must: Recalculate what they owe for days 1–12 on the lower plan Issue a credit for the overpayment Apply that credit to the next invoice (not a separate line item or manual journal entry) Each of these is arithmetic; the hard part is automation and data coherence. A weak tool leaves you hand-correcting invoices or losing the audit trail. FreshBooks: Simple plans, fast proration—and a ceiling FreshBooks handles mid-cycle plan changes better than Wave for straightforward recurring invoicing. When you upgrade a customer mid-month, FreshBooks calculates the daily rate automatically and issues a single invoice for the prorated difference. The invoice date aligns with the change date, keeping cash flow reports clean. Where FreshBooks works: Tiered plan upgrades (moving between fixed monthly tiers) prorate cleanly Credit application to the next invoice happens automatically; no manual journal entry needed Multi-currency invoices handle proration at the current exchange rate Recurring invoice templates let you set proration rules per customer (some allow partial refunds; others rollover credits) Where FreshBooks breaks: Usage-based overages require manual line items. If you have 100 customers with variable API usage, you'll either meter overages externally and import them, or hand-code them into invoices each cycle No native integration with metering platforms (Stripe Metering, Recurly, or custom APIs). You'll use Zapier or a webhook script to push overage data, adding latency and failure points Multi-unit or dynamic pricing (e.g., "$5 per user for the first 10, $4 for 11–50") requires manual recalculation Refund reversals create separate credit memos, not rolled credits. Your accounting sync (to Xero, QuickBooks, etc.) treats them as separate line items, muddying the books FreshBooks works well for flat-rate recurring invoicing with occasional mid-month plan swaps. If your billing model is fixed-tier subscriptions (Starter → Pro → Enterprise), proration is automatic and clean. If you need metered usage or dynamic pricing, you're building a bridge. Wave: Free, but proration is manual Wave is built for small agencies and freelancers sending invoices, not managing recurring billing at scale. It does offer recurring invoice templates, but proration is not native. What Wave offers: Recurring invoice creation with date-based repetition (every 1st, every month, etc.) Line-item editing on each generated invoice Free accounting sync to Wave's own ledger The proration problem: When a mid-cycle change occurs, Wave doesn't calculate or suggest a prorated amount. You manually edit the next invoice or create a one-off invoice for the difference. At 5 customers, this is tolerable. At 50, it's a disaster. Wave has no usage metering, no overage automation, and no credit application logic. You'll be hand-editing invoices or scripting a Zapier workflow to do the math outside Wave and push corrected line items back in. Wave is not a billing platform; it's an invoicing form. If proration happens, it happens in your spreadsheet first, then you type the number into Wave. Str