The green tokens are used when using the green archer dice from Dark Mountains expansion.
You may turn the tokens in for Valhalla rewards as soon as you can afford some, but it has to be done as the last step of of any combat you resolve.
You do not refresh the available cards until you have finished spending the tokens, however, so you can only choose from what is available when you start purchasing them.
Hope this helps.
Ah..makes sense. I don't have the dark mountain expansion. lol Very helpful thanks.