For several years before Prime, Amazon offered free two-day shipping on most $25 orders, no extra fees or membership. Maybe it started out at $50.
Remember that way back when it was business as usual to pay $10+ for shipping if you bought online. This meant that if you bought online more than 4 times throughout the course of an entire year, you were ahead. It also allowed you to buy at any time without worrying about the minimum purchase total, increasing buying frequency.