Invoicing in one currency (CRC), but charging cards thru gateway in USD?

Hi all!

Here in Costa Rica, by law all legal invoices must be written a) in Spanish, and b) in the local currency (Costa Rican Colones).

I quote, invoice and bill in US Dollars, and that's beneficial to me because I use PayPal Payments Pro as my payment gateway and as far as I know, CRC's aren't a supported billing currency for PPP.

However, what I need is for my invoices to do one of the following (so I can make them legal):

a) Invoice all lines, and tax calculations in CRC, give a total in CRC, and then have another line saying what the USD equivelant is and have it charge that through the payment gateway, or

b) Invoice all lines, in USD, but for taxes and total give a converted rate in CRC, but still process thru PPP as USD.

The currency is generally fairly stable at 500CRC = 1USD, so I don't necessarily need auto-updating currencies to happen.

Any ideas on how I can make this happen, or how it has been implemented in the past? I've looked through here and haven't found a proper way to do it :(

Many thanks to all!
Cody