How did she “accidentally” receive some of Grassley’s mail?
May I speculate?
She 'accidentally' paid the person who does the hand delivery of the invitations to drop it off at her office first.
Her and her cohorts had to have been working on arrangements for this gig, so they could have simply heard, or had some insider provide them with a hot lead. All they needed was the written invitation to use as proof.
She accidentally received some of his mail like she accidentally lost her emails from her hard drive (which isn't even where they are kept).