You may still be able to get an official Apple battery repair through an Apple Authorized Service Provider (not sure about an Apple Store). Apple's details for supporting battery replacements for up to 10 years is very vague and does mention "while parts are still available" which makes it even harder to understand.
Trying to do the battery replacement yourself on a 2015 model is very tricky especially with the expanded battery cells. I have seen numerous reports on this forum where people have accidentally damaged their laptops during the procedure. If you are good with repairs in general, then you should be able to handle the repair if you take things slowly and are careful. The adhesive softener provided in the kit should do a good job of loosening the adhesive on the battery (at least the iFixIt kit does, haven't used an OWC kit yet, but will on the next one soon), but it still takes a bit of effort to separate the battery. Things get tricky after loosening the first cell since it starts to flop around as you move the laptop to get to the next cell. Watch the video instructions OWC provides on their website for the battery replacement kit to see what is involved. Keep in mind, it will be a bit more difficult than it looks in the video....they make it look really easy. OWC does recommend professional installation.
I cannot be sure whether you have the 13" or 15" model. If you have the 15" model, then it is a major repair if you remove the speakers as instructed since the speakers fit beneath the Logic Board....meaning everything must be removed from the laptop in order to remove the speakers in order to give you more room to remove the battery.
You do need to be extremely careful when disconnecting the speakers from the Logic Board since the speaker connection can be quite tight sometimes.....especially on the 15" model. I have actually lifted the speaker connector from the Logic Board on the 15" model even when I'm extremely careful & am already aware of it being an issue.