This addon allows a second currency to be displayed while still sending the default currency at checkout. This is important since some payment modules require payment in US funds. To accomplish that, the currency in the shop must be set to default currency prior to confirming the order, which makes it difficult for customers from different countries to realize what they are actually paying. With this addon, both currencies can be displayed at various places in the shop, depending upon an admin setting, so that the customer always knows what he is paying. You can see this in operation here since we use it ourselves. Just switch the currency using the above selector, switch to a different product so the option can be set and notice how both currencies are shown.
It is recommended that the Automatic Update Currency contribution also be installed since currency rates fluctuate daily.