You are right. I waste $100
It would appear that you’ve wasted the $100 if you can’t get a refund (I doubt that you can), but in spite of that, it’ll be easier to use the free cert than using the one you paid for.
I am seeing mix content now that I enabled Let’s Encrypt. Where do I find what to change to make everything https://?
Visit Admin -> Settings and search for ‘https’ - make sure ‘Force HTTPS’ is turned on so that all local URLs are rewritten to reference the secure path.
If you have any customisations which explicitly reference off-server HTTP resources they may need to be manually corrected.