It may vary, but from my experience you can let them know what cert. you want or don't want. For instance, if you absolutely don't want Shuttle, you can tell them and they will let you know when they have CHQ or REP classes starting, and vice versa. You can decline a class offer 3 times before they give up on you. The HR people, from my experience, are all super friendly and very helpful. Just let them know what you want or don't want and they will probably accommodate.