Currency handling is consistent

Floating-point arithmetic on monetary amounts produces silent rounding errors — 0.1 + 0.2 is 0.30000000000000004 in JavaScript. When tax, discounts, or proration are calculated using float dollars instead of integer cents, the errors compound across transactions. CWE-681 (Incorrect Conversion Between Numeric Types) covers this, and ISO 25010 functional-correctness requires that calculations produce accurate results. Formatting currency with string concatenation ("$" + amount) produces locale-incorrect output (wrong decimal separators, missing grouping) and breaks for non-USD currencies.

Store all monetary amounts as integers in the smallest currency unit (cents for USD). Use Intl.NumberFormat for display and integer arithmetic for calculations.

// Store amounts as cents (integer)
const amountCents = 2999 // $29.99 — never store as 29.99

// Display with locale-aware formatting
const display = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',
}).format(amountCents / 100) // "$29.99"

// Arithmetic stays in integer cents
const taxCents = Math.round(amountCents * 0.08) // 240 — not 2.392...
const totalCents = amountCents + taxCents        // 3239 — exact

Stripe's API returns amounts in the smallest currency unit by default — work in those units end-to-end and only convert to display format at the UI layer.