What to do with that leftover currency one has after an international trip? There are receptacles for it at some very large airports, where it’s donated to charity, but those aren’t prevalent at all. Aside from that, there’s not much. Keep it as a memento? Throw it away? Maybe, but that can very quickly become expensive.
How about a mail-in currency exchange? On the business end, it could be run from a website that lists current exchange rates, the percentage or flat fee charged to do the exchange, and a customer service interface that keeps track of amounts and addresses. On the customer end, all one would have to do log into an account, let the system know how much is coming from what address, stick the money in an envelope and send it. Once it arrived at the business, it could be combined with all the other incoming cash to make a deposit big enough that the cost of exchanging it would be smaller than the value of the currency.
There are companies that do this as part of their larger currency exchange operations, but as far as I know none accept coins, and most require minimum amounts of foreign currency. I’m sure the reason for that is that they just don’t want to have to deal with such small amounts, or don’t think they’ll be profitable. But if a business specialized in those amounts, it could work.
One final thought: to increase profitability, the business could, instead of incurring the expense to exchange and ship currency back to the customer, pledge to donate extremely small amounts (minus a small fee) to charity. Many people, including me, would pay to send their leftover international cash somewhere, if they knew it would be going to a good cause.