Old battery was dying out so I decided to order a new one from ifixit.com.
Order came through in couple of day, old battery out - (easy), new battery in (easy) and power on.... Problem. The new battery was faulty and did not work. So I contacted ifixit.com support team and explained the situation. Had some advise, downloaded a battery program from coconut and sent the details to support. They also soon deemed that the battery was faulty and sent me a new one. I did not have to sent the faulty one back and saved a lot of hassle. Second new battery came in, at this point I was already a pro in opening my MacBook Air 13" back cover and I changed the battery in less that 5 minutes. All good.
Ordering from an online store and getting a faulty product is the thing that worries me. Are they going to replace the product, how long does it take to send a new one, are they going to charge more etc etc.
I have to say that ifixit.com handled this thing 100% and are very trustworthy company. Just usual checks from them that it was indeed faulty battery, after that they were quick to replace without extra cost and extra hassle. I will be using them again in the very near future. You've made me your loyal customer.