securing access to an app engine service

[krem]

I have a GAE java service with the following access points:getCost(id)setCost(id,newCost)on domain: myService.appspot.comI also have a front end php app on: frontend.different-domain.comthat will need to be able to access those two end points. getCost can be publicly accessible by anyone but I was wondering how I could secure the setCost() endpoint to limit it to only requests coming from frontend.different-domain.com I'd like to avoid random people calling setCost and changing the price of things.thanks