Sending individual dollars by post might be a bit awkward. But here's another idea. If all those who are passing through Madrid, say, every month, get together for a few drinks, we'll buy you a beer. Since you're not here to drink it, we'll just have to put the money in a hat - el bote - and send it to you now and again. This should be a fun way of raising a bit of money and honouring/drinking to your health at the same time.