Can some people please give me some opinions on how they think the following situation should be resolved?

I had an electronic component where the end of the case had broken off. The damage was caused by normal usage and not the result of crash damage or mishandling. I emailed the manufacturer to see if they could send me a new case and they said that it would probably need to go back for servicing. They also asked me to forward a photo of the damage which I did.

I packed up the component and was just about to ship it when I received a new case through the post. I was a bit surprised as I wasn't expecting it but was quite happy to be able to fix the unit myself. Having changed cases on receivers, servos and ECUs before I wasn't worried. That was, until tried to take the old case off. What the manufacturer hadn't told me was that some of the electronic components were actually glued to the inside of the case and there was no way I was going to be able to remove it without damage. As a result I ended up breaking several of the components away from the PCB.

I thought that it would now be be beyond repair but I sent it back anyway, they have fixed it, and I have just received a bill for the work. Do you think this is fair?

Is it my fault for causing the damage or should the manufacturer take some of the responsibility for supplying me with a component (the case) that wasn't simple for me to replace? Had I known what might happen, I would have sent it to them to do for me. At best now, it seems that I will have to pay to get my repaired unit back when it only had a damaged case to start with[&o] I'm not a great fan of using repaired items in my jets so it is now almost worthless to me.[&o]