Hi amin!
I have this same problem as well, but it’s a tad more odd than others.
It used to work fine (and works fine on localhost), but suddenly it stopped passing the session values through paypal.
Try it out here : http://test.wanderlustandco.com/shop/oh-deer-green-ring/
It’s totally fine when you’re browsing around the site though, and I don’t have any caching turned on.
Would this still 100% be a problem that hosting can fix?