You can configure the currencies you wish to use in your WHMCS installation in Setup > Payments > Currencies
By default the base currency will be USD. This can be edited and should be changed to your home currency if not USD. You can then add additional currencies to run alongside that.
Currency Setup Guide:
Change Default Currency:
Adding/Editing a Currency
The fields for adding or editing a currency are the same. Below is an explanation of each of these:
- Currency Code – This should be the standard 3 letter ISO 4217 code used by gateways
- Prefix – The text/symbol to display before the amount
- Suffix – The text/symbol to display after the amount
- Format – Allows you to choose between the different format styles for currencies
- Base Conv. Rate – The rate amounts in this currency should be divided by to get to the base currency, eg. USD to GBP currently 1.5
To find the base conversion rate; visit The Universal Currency Converter, select your default currency in the "From this currency" menu, then select your second currency from the "To this currency" menu and click Go. The base conversion rate will then be displayed as highlighted below:
The first currency is the default one. If you want to define a default currency you must to edit the currency code, and set it to the new default currency.
Auto Updating Rates
WHMCS has the built in capability to retrieve and update the base conversion rate values daily for the following currencies:
USD, EUR, JPY, BGN, CZK, DKK, EEK, GBP, HUF, LTL, LVL, PLN, RON, SEK, CHF, NOK, HRK, RUB, TRY, AUD, BRL, CAD, CNY, HKD, IDR, INR, KRW, MXN, MYR, NZD, PHP, SGD, THB & ZAR
For a full list of international currency codes: 
You can manually request the rates to be synced using the Update Exchange Rates button on the currencies page or you can configure it to be automated with the daily cron in Setup > Automation Settings.
Auto Updating Prices
WHMCS also provides the ability to have the prices in currencies other than the base rate automatically updated based on the rates set. Again you can invoke that manually from the currencies page with the “Update Product Prices” button or you can have it run after the daily rates update in Setup > Automation Settings.
Specifying Currency in a link
You can link to the order form with a specific currency by simulating what the form shown for currency selection does - eg. cart.php?currency=1 cart.php?currency=2