Battery changing

I live in Turkey but I bought my Phone ( iPhone 6s plus) from abroad three years ago. I want to change my phone’s battery two weeks ago but also I lost my phone invoice. When I went to the technical service they said that we can not change your battery because you bought your phone from another country and also you lost your invoice. Is this normal? So that I had to changed my phone’s battery in the unauthorized service. And I am being sad about that.