Summary: The cost of the battery is around 85 dollars. It takes an experienced person 2 hours to change the battery. If someone charges under $100 for labor, it is a very good price. It should be done in 2-3 days.
I recently had the experience of changing the battery of my 2015 Gold 12" MacBook. After 5 years, the battery can only last 20 min on a full charge. It is very similar to a MacBook air, and had 13" and 15" cousins. It had a similar appearance to the MacBook Air, but was thinner, lighter, and had more storage and memory. I am happy with it and don't want it go to the laptop grave yard in my basement. You can look up the model number, EMC number and serial number at the back of the machine. They are marked in plain English.
RULE #1: Don't try this at home
Apple did a super duper job of compact integration engineering. It is difficult to untangle all the screws, parts to reveal the battery. Battery is serious business since heat condition and electrical connection must be right. So I used the google search term "Mac book repair" to find the LOCAL REPAIRMEN WITH PHYSICAL ADDRESS OF BUSINESS. This search term is the best.
“A1534 EMC2746 battery can be changed for $85 part cost and $60-100 labor cost, and it should take less than a week.”
What worked for me
I eventually learned to search under "MacBook repair". This search term is golden at revealing the right source of repair resources. I ended up contacting Fast Geekz, a one-man operation in Chicago. The guy did a fantastic job. Other vendors offered to do the job locally for 250 dollars which is also reasonable. (Don't try to national chains that advertise on Google - they all do this by shipping your computer off state).
What I tried and found did not work
I tried a number of candidate sources - local APPLE store, Geek Squad of local BestBuy store, and some "local" repair men through google search. The APPLE store takes 2 weeks and will undoubtedly change an arm and a leg for this job. I did not pursue further. The Geek Squad team does not do the job locally and must ship it out (perhaps to somewhere in California). Local repair person are sometimes overcharging (250 dollars) or does not appear to be confident in handling this particular change.
Of course I tried to see if I could repair it myself. There are DIY videos on youtube, such as this one linked below. It looks like an average electronics green thumb will have a lot to learn and practice. I might take too long, and DIY is not worth it unless you have opened at least iPhone or iPad before.
You can go to youtube and search with MacBook Air battery change or MacBook A1534 battery change. (IF you wish to do this on your own and buy a battery from Amazon, make sure you buy it from a reliable source. Batteries can damage a lot of things if handled wrong.
The following are pictures of the MacBook machine in various stages of disassembly.
In case you want to try DIY battery change on A1534 EMC2746, you need to have special screw drivers such as Pentalobe and Torx drivers. You can buy a good set on Amazon for only $10.
Good luck with renewing the life of your favorite Mac - the 2015 MacBook A1534. Of course, this information can also apply to other Apple MacBook and MacBook Air models.
Add a Closing
It took me a while to happily fix my low battery problem of my 2015 MacBook air. I learned that the year, the model and EMC models are important since Apple was actively engineering a lot of models at around the time. After trying Apple Store, BestBuy and some expensive local repairmen who does not have the right tools, I searched with Google search term of "MacBook Repair" and yielded gold. I suggest you can do this from your physical location as well.
Apple charges a lot and takes a long time to do it off site. Find a good person, or contact someone who "does this for a living" and do a honest job.