Multi-Currency POS & Invoicing Architecture: Designing for USD, ZiG & Rand Volatility

CREATE TABLE invoices (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  invoice_number VARCHAR(32) UNIQUE NOT NULL,
  customer_id UUID REFERENCES customers(id),
  
  -- Base Currency (USD cents)
  base_currency VARCHAR(3) DEFAULT 'USD',
  base_subtotal_cents BIGINT NOT NULL,
  base_vat_cents BIGINT NOT NULL,
  base_total_cents BIGINT NOT NULL,
  
  -- Settlement Currency (e.g. ZiG, ZAR)
  settlement_currency VARCHAR(3) NOT NULL,
  exchange_rate NUMERIC(18, 6) NOT NULL, -- Rate at exact transaction moment
  settlement_total_cents BIGINT NOT NULL,
  
  status VARCHAR(16) NOT NULL, -- 'PAID', 'PENDING'
  created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
);