If you were planning on inviting her, still invite her. Just let the hostess know to please put a note in her invite "I've been informed you have already given the MTB a generous gift, please feel no obligation to get anything further" or something like that. Or you could just say it to her.
And then at the shower when its gift opening time, just announce "and Mary gave a gift car last month to Awesome store!" with a big smile and let her get her little bit of smiles and maybe a smattering of applause or however people react to gifts in your circles. Most likely you will get a few other gifts shipped to you, or given early so this will be a very normal thing for you to say. (I have never been to a shower where the guest of honor didn't mention gifts given outside the shower.)