# ExchangeRateProvider

## Import

```typescript
import { ExchangeRateProvider } from "@alepha/commerce/invoicing";
```

## Overview

What one unit of a currency was worth on a day.

Prices never read this: a price list is written by hand, and a sale is billed
in the currency it was made in. It exists for one line of an invoice. An EU
invoice may be in any currency, but the VAT payable has to appear in the
seller's own as well (article 230 of the VAT directive), at a rate the tax
administration accepts.

Which source that is, is the seller's call: the ECB's daily reference rate, the
customs rate of the month. So the default knows none, and an invoice issued
without one goes out without the line and says so in the log. Substitute this
class before selling with VAT in a foreign currency.

```ts
alepha.with({ provide: ExchangeRateProvider, use: EcbRates });
```