Okay. This is Not Good and she needs to know that. But for your own sanity preservation, remind yourself (not her, obv) that it is (only) a property crime, she hasn't attacked anybody, conducted a cyberbullying campaign or been secretly taking drugs, so it's not the end of the world as we know it.
I'd guess the reason she's been doing it is because she can the shop is there, she passes it regularly and she's got away with it once so why not keep going. It's presumably only partly about the actual earrings if she really really longed for particular earrings she'd have nicked a few pairs, but not that many. Maybe she's getting a buzz out of it? Does she have friends who might be encouraging her or daring her to do it? You do get an adrenaline high from shoplifting [memories of teenage attempts to nick picknmix from Woolies, and Rimmel lipsticks from Boots...]. Does she get pocket money/allowance? If she had really wanted earrings, could she have bought them herself, or asked you to get them? Is she desperate to get her ears pierced but hasn't found a way of talking to you about it? I think if you can try and figure out what's going on in her head (and sometimes that's really hard, lol) it will help you decide how to play it.
Assuming that she does have the means to buy stuff occasionally (ie. she hasn't nicked them just because she has no legitimate means of getting her hands on cool stuff she wants) and she isn't being pressured or bullied into it, then I'd probably do something like this (my dd1 is 14 btw):
DON'T say anything in the car. Get her home, do cup of tea, whatever, make sure there aren't any brothers or sisters around. Then do the "I need to have a talk with you" routine, tell her what you've found and ask her to tell you what's going on. Don't enter into discussions if she comes up with daft excuses and don't get angry, just be serious. Explain that it's stealing, it's a crime, she could get a criminal record which might prevent her getting a good job (or even a holiday job). Whatever it was that made her do it (wanting the earrings, wanting her ears pierced, being bored, wanting to show off to her friends, whatever), stealing is never a good solution to that. Keep repeating that if she wants stuff she needs to come to you and talk about it sensibly.
I would seriously consider making her take it back to the shop and apologise to the shopkeeper, but you would need to play that one by ear. I don't think she'd get into trouble with the law for returning stuff she'd nicked, though presumably the shop staff will be keeping a v. close eye on her in the future, which could work to your advantage.
And don't panic, teenagers and kids nicking things from shops is incredibly common. There's no point going nuclear, but you do need to make it very clear that it's not okay and mustn't happen again.