FoxyInvoice Dev Blog: How Invoice Lifecycle, Tax Engine, and Exports Were Built

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Developer blog series FoxyInvoice has published Chapter 5, detailing the domain logic behind its invoicing platform built for real production use. The invoice lifecycle follows a state machine moving from Draft through Sent to Paid, Partial, Overdue, or Void, with overdue status derived daily rather than stored. A dedicated tax engine handles US sales tax complexity by evaluating exemptions, nexus, jurisdiction rates, and product taxability per line item, using decimal arithmetic to avoid floating-point errors. All totals are computed server-side, with the browser's figures never trusted, and invoices lock after a configurable window to prevent retroactive edits. The platform also supports export formats including CSV, QuickBooks IIF, and Tally XML to meet accountant requirements across different markets.

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