The site is a honeycomb of 54 cells. Pick the one your trees will grow in — your certificate is numbered after it, so the number tells you exactly where to find them. Cells outlined in green are available.
All four fields are needed: the certificate is printed from them and the tree's letter is sent to them.
The secure payment page is opening in a new tab. Pay there — by card or by scanning the QR with your banking app. That tab closes on its own the moment the payment clears, and your certificate appears here.
Keep this tab open.
Thank you for your contribution to the fight against desertification.