BOGO Baby: The Blog began as a treatise on how to save money with twins (or more). I recently heard about a group Mothers of Super Twins, "founded in 1987...the leading national nonprofit provider of support, education and research on higher-order multiple births."
They provide this handy list of company freebies and discounts for multiples families. Shoes, formula, portraits, various toy companies.... the list is long. The site includes a sample letter for you to copy and amend for your needs.
They also have a list of freebie sites, coupons, articles, and more for mothers of multiples.