Amazon has it's least expensive Kindle on sale today for just $49, normally $69. This is the no-frills straight-up e-reader. It's 6" tall and will hold 1,000 books. It comes with special offers, so that means some ads. However, my SIL has one with special offers and doesn't find the ads intrusive at all.
gift for under $50, this could be the one! It ships free with Prime, but should qualify under the $35 shipping rule as well.