This is 100% possible to do without any apps. It should be very easy to setup. Can you share a screenshot of your current shipping settings? It should look something like this (if you’re doing it correctly):
I realised my first problem - I had the free shipping as $501 or more but the cart was $500, so I fixed that but when I go over the $500, 2 options for shipping appear - Standard shipping at $20 and then Free Shipping.
Those screenshots look good/correct. It’s normal behaviour for the 2 different rates to show up. There’s no way to get the weight-based rate to not show up if the price is over a certain amount. It’ll show both rates, but it should pre-select the cheapest rate (ie. the free one).