Isight camera and iphone 6 plus

I've observed that the front camera on my iphone 6 plus does not takes proper in-focus Pictures. The unit have a Serial number that is qualified for replacement/repair of camera. Ref: Erstatningsprogram for iSight-kamera på iPhone 6 Plus – Apple-kundestøtte

After finding an authorized Apple vendor, I visited Elkjøp Stavanger, (Norway) service centre and explained the issue. The service centre asked and required the invoice of my 3 year old iphone. They were not able to give any free service without my invoice, that I'm not able to find. What a service! They suggested that I should bring this issue up with Apple. Are Elkjøp really an Authorized Apple vendor and what responsibilties do such company have? I think, I'm treated bad and given a poor service. I want to tell about this experience, and both Elkjøp and Apple needs to improve in this matter.