My daughter has a Macbook that is 4 years old and the battery has stopped charging. Looking on-line at Apple, they want ~$129 or so for a new one and an appointment to replace it (

). Reviews on Apple say that don't last as long and have swelling issues.
Customer Reviews: Rechargeable Battery - 13-inch Mac
I've looked at hers and it pops out like any other battery, so replacing it myself will be a snap.
Elsewhere on-line, it $30 to 50 for a new one.
How do I test if it's the battery or something else?
Anybody replace theirs and have good/bad experiences, plus cost?
BTW, I'm not a Apple/macbook user so it will be difficult to diagnose, as I'm computer impaired.