What if the girl they sent the rec for was a real hosebeast and the chapter spent the whole time of rush praying she'd drop them so they didn't have to do it? Just having some fun with the question
But seriously, I would think an email would more than suffice - the rush chair could just write a generic "thank you for your rec" email and blind CC it to everyone who sent recs. The thought of those poor girls having to fill out 1500 postcards is just dreadful.