so now with the monthly plans I can just buy it when I need it and immediately disable auto-reneweal

I’ve been doing this for Amazon Prime. I shop there as infrequently as I can manage, but sometimes it’s just the best way to get something I need. In those cases, when I check out, they invariably offer me 15 days of Prime for like, $4.99 or something. So I accept the offer. Place the order, then later that day, cancel the Prime subscription. They issue a pro-rated refund, so I end up paying about $0.35 for 2-day shipping on any order.