Apple ripped me off - can I do anything?

Hi, so last November I purchased a used iPhone 6s to replace my previous iPhone 5 that would no longer charge and died on me. I got what I thought was a good deal on the phone, but realized shortly after getting it that the lower price was probably due to the battery health showing 81% which upon research, meant it was close to needing to be replaced. Luckily, or so I thought, my research also revealed that Apple had reached a settlement about deceiving people about batteries (how ironic) and was offering to replace batteries out of warranty for $35 dollars until Dec 31st, 2019.

It seemed like a great opportunity so on Dec 22nd, I brought my phone in to the Apple store, Rideau Centre in Ottawa, and ended up leaving my phone with them to get a new battery. A few days later, they said it was ready and my girlfriend picked it up and paid the bill ($39.55). When I got my phone, I checked the battery health, and it still said 80%, which I found odd. I thought maybe it took a little while for it to update after a new battery was installed. But, it never did.

My battery health is now 'Service' even though I paid Apple for a new battery less than a year ago.

I did call Apple support and spoke to someone about it who said records showed the work was done but I'd have to bring it back to the store to have it looked it. Unfortunately, it is extremely inconvenient for me to go to the Apple Store, and instead I have just let it stew in my soul instead of taking further action in the last 10 months.

I know work is guaranteed for 90 days or something, but in this case I don't think it applies anyway since the work was never done. I believe this is actually theft.

So is there anything I can do? Is there any chance Apple would admit this mistake and fix it or did I just get taken advantage of and there's no hope of retro-action now?

I'd appreciate any input/help as I have been paralyzed by this for too long now.