SP3 Service failed to fix anything

I recently sent in my SP3 for repairs. The problem was that the battery never charges to 100%. It always stops around 85% and it just says "plugged in, not charging". so i called microsoft and sent in my surface to get it fix, going by the standard exchange (because apparently advanced exchange was not an option anymore in Canada). About 2 weeks later they sent my broken unit back saying there was "damage by any external cause", which there was NOT. Nor water damage or missing parts. I had a friend who send his surface in with a cracked screen and got it replaced with a brand new one. I am extremely unhappy. Has anyone experienced this and any suggestions?