To my astonishment, the answer: 1999.
I placed that order at www.discountblvd.com, according to the email receipt. They went out of business a few years later — as did many of the pioneers in online shopping. One month later in 1999, I ordered a pre-release Steely Dan CD from a vendor that was new to me, Amazon. About 400 email receipts from Amazon are filed in the "eCommerce" folder on my laptop. Buying from Amazon is now a weekly event, if not more often, in this household. The most recent order was for Tamanishiki rice, a fabulous product from California that we get in four-pound sacks.
Of course, Amazon is not the only online merchant I buy from. Some of the others: 1000bulbs.com, eBay, Big Tall Direct, StubHub, Ticketmaster, Dell, OfficeMax, Woot, LL Bean, JC Whitney, Penneys, Papa John's, USPS, and Joseph A Banks.
My first online order was a vanilla-flavored tea. I don't remember the name of the underlying merchant, only that they were located in Toronto. The order went through the Prodigy portal. This would have been 1990 or thereabouts; Prodigy had just released a client for the Macintosh, and in those days we were a Mac household. The tea was excellent but its price on Prodigy was outrageous. I did the order for the novelty of it. One of Prodigy's big mistakes, as it turns out, was an assumption that online shoppers would pay premium prices for rare items because of the convenience of online shopping. When Jeff Bezos started Amazon, he took the opposite view: online shopping should offer lower prices across a broad inventory. Books and CDs were merely his entry point. Prodigy dwindled to nothing, and Amazon is worth $400 billion. Sam Walton had the same idea: price does matter.