If I understand your API correctly, the call is doing a change in the resource at the server, this means that you should use a POST:
Per RFC 2616 , the POST method should be used for any context in
which a request is non-idempotent: that is, it causes a change in
server state each time it is performed, such as submitting a comment
to a blog post or voting in an online poll. In practice, GET is often
reserved, not simply for idempotent actions, but for nullipotent ones,
ones with no side-effects (in contrast to "no side effects on second
or future requests" as with idempotent operations)
http://en.wikipedia.org/wiki/POST_%28HTTP%29
http://en.wikipedia.org/wiki/Http
This is, in my opinion, a very important point between the difference between GET and POST. POST is not only there to allow you to send information in the body, but also to modify the state of the server. I always have it in mind when designing REST interfaces.
I hope it helps.