u can not get tax back at airport. u can only get form there. (most hotels have the form also). wnat to do is mail all recept and the form from your foreign address and copy of your visa. ( in this case. if you are a PR. u cann ot claim that).
and I remember that the 50$ is the tax not the purchurse value. eg. if the tax is over 50$. you can claim a return.
try to get a form at any hotel or ott. air port . there is a detailed explaination.