Which retail innovation of the 1930s first necessitated 'eye-catching' product packaging designed to sell itself from store shelves?

Before the 1930s, grocery shopping was a completely different experience from what we know today. Products were typically sold in bulk from barrels or bins, with shopkeepers measuring and packaging items for each customer. This poll explores the historical innovation that revolutionized retail by introducing self-service shopping and fundamentally changing how products were packaged and marketed to consumers.

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The self-service supermarket (where Piggly Wiggly and King Kullen pioneered customers selecting their own products)
Mail-order catalogs (where Montgomery Ward and Sears required distinctive packaging for shipping)
Automated vending machines (where products needed standardized packaging to fit dispensing mechanisms)
Department store display windows (where luxury goods packaging became an extension of the shopping experience)