As per one of my recent posts:
I've been involved in writing many major international contracts in the past and use "shall ", "will" and "may", with very specific meanings, on a very regular basis - as Jose says, these are more formal situations.
The guidelines we have used are:
"shall" indicates that the Contractor shall (i.e. must) do X.
"will" indicates what we, as the client will do (because it's what we always do in such circumstances),
"may" indicates an option where the Contractor (or we) may do A or B depending on the circumstances at the time.
PS We already know they "can", otherwise we wouldn't have awarded them the contract!