Hi, my site allows users (marathon runners) to create a marathon profile page so that they can accept donations from their friends and families.

I have a section on the site called Charities, which shows the amount of money that each charity has received through the site.

The site currently allows users (donators) to donate in USD, EUR, GBP & AUD so each marathon profile page could have a mixture of donations from a mixture of currencies.

There are 30+ charities on my site that the marathon runners can choose to donate their money to at present.

Question
How do i control / calculate the overall total raised for a single charity as there will be a mixture of donations / currencies. A user can choose one of USD, EUR, GBP & AUD as their default currency view in their account, so i can just display the total in one of those, but how do i add all of the donations up.

I was thinking maybe it would be best to just convert all donations for the chosen charity into the users default currency using an exchange rate for each of the 4 available currencies and then manually changing the exchange rates on a daily basis (or pulling them from some exchange rate site)

How do you think this would be best handles, thanks in advance for your advice...