I think I'm a few months ahead of you. I'm still ok but I can definitely see the deterioration in the battery life.
The official answer is to send Apple $90 and your phone but I can't imagine a lot of people are doing that. I can't be without my phone for that long. You can find a battery and toolkit for $10-$13 and change it yourself. That would void the warranty but I'm pretty sure I'm out warranty anyway since I didn't purchase the extended.
I’d change it myself before I send it off but I’d prefer to take it to somebody local.
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