I'm thinking of using the Google Endpoint service to add some simple features on my Android app. Following a tutorial on Github (https://github.com/GoogleCloudPlatform/gradle-appengine-templates/tree/master/HelloWorld) I managed to create a very simple servlet with basic code and that's all I need. But it's not clear if I'll have to pay at some point (30 days free trial?) or if I only have to pay to access to the whole platform.. Is a simple servlet on Google Endpoints completely free without restrictions?
It depends on where you are hosting your servlet. If it e.g. meets the constraints for Google App Engine's free quota you won't have to pay (as far as I know). I am in fact running such an application since quite a while now.