Correct me if I'm wrong because I'm viewing on mobile via flaky internet connection but it looks like the $49 minimum purchase for free shipping with Prime is currently gone.
Eg 2 gallons of Elmer's glue (probably worthy of a seperate bargain https://www.amazon.com.au/dp/B002FTOBZE/ref=cm_sw_r_cp_api_i… )
Edit: terms and conditions thanks to Selenium. "Limited time only" so get in quick
https://www.amazon.com.au/b?node=7575290051
When I first signed up for Prime I thought the same as you - that it will be easy to find $50 worth of stuff anytime I want to place an order and so free shipping for orders over $49 will be almost as good as free shipping for orders of any value.
In practice I found that I rapidly bought everything I was in any hurry for and now have tons of things on watch lists waiting for a particularly good deal to turn up. I'll often find something has dropped to a really great price only to realise that there's just nothing else worth purchasing with it to make up the $49 required for free shipping.
The minimum spend is especially annoying given that there doesn't seem to be much of a reason for it - almost everything seems to end up shipping separately anyway so it's not like Amazon is saving money by combining items into a single shipment.