You can select an alternative currency to display on your invoice in addition to the billing currency you've selected. To select/edit this alternative currency, navigate to Billing and Payments from the profile drop-down. On the Billing Details tab, scroll to the bottom and select a currency from the drop-down.

Note that once your billing details have been saved, you can no longer edit the billing currency. However, you can edit the alternative invoice display currency at any stage.

If you are currently using the old user portal:

Your alternative invoice display currency is an alternative currency that you can choose on your Transaction history. This alternative currency will then display on your invoices in addition to your billing currency.