To save on shipping costs, you can remove the film canister from the disposable camera before mailing it in. All disposable cameras are slightly different, but the process of removing the film is similar between each type.
After you finish taking pictures with the camera, continue winding it until it all goes into the film canister. Next, Remove any plastic or cardboard wrapping from the camera.
At this point you should see something like this:
Open the door, and you should be able to pull out the film canister
The film will slide right out