Used stamps exchanges.

I just received some stamps with a UA id which I registered. I need to send stamps back obviously but the system doesn't give me an id to put on the envelope. Any ideas?

than you have to wait, so the recipient gets stamps and than you will be able to send another lot
more you send, more open spots you get...

It says that I have reached the maximum number of exchanges.

You don't have to send stamps back!
Just click the button "Send stamps" and see what happens :)