More damage than good

I've booked my phone in for repairs. All that was damaged was the led. Once it was repaired it was sent back to me. It's been a few months since the repair. Since it's got back the battery life seemed so poor. I ignored it put on the power saver and continued but in these last few months it's gotten so much worse where the phone switches off. Doesn't charge. And when I take it back to the service provider the contract is with they say there's nothing more they can do and I'd be liable for charges. How is this my fault? We send it to the company it belongs to trusting the job will be a hundred percent and now we liable for charges!