Libera posted the following to their blog today:
We want bring our music to people at affordable prices, but this means that we do not cover all our costs through ticket sales and free will offerings. As a touring choir we have some quite considerable expenses to cover - for instance our visas to visit the USA for our upcoming tour cost nearly $13,000 alone.
We do not have a large company or commercial backer in the background paying the bills, which means we have to raise our own funding. So if you feel able to help us cover our costs by making a donation, please visit www.justgiving.com/libera/donate.
If you wish to make a more specific offer of financial support, please feel free to contact us at email@example.com.
I did not realize it costs so much to make the trip to the U.S.! Please help if you are able.