We are using TestFlight for beta testing and we have an IAP store in our app.
Will the Beta Testers be able to buy the packages from the sandbox environment?
If the Beta Testers have a proper development provisioning profile installed on their device, then yes.
For development versions of the app, the app will run in the Sandbox environment for IAPS, Game Center,.. and the beta testers will be able to buy the in-app purchase by logging in as a test user associated with the iTunes Connect account(which you need to create first under the "Users - Test Users" tab within iTunes Connect).