Where possible I always recommend you buy the New glass from me and install it yourself. You only need a Soldering Iron and basic tools eg flat head screwdriver and Craft Knife. You only need to Solder two wires and it doesn't need to be a 'pretty' solder joint as they are hidden inside the mirror. Nobody will see your embarrassing blobs of solder when you put the mirror back together!
I do offer a repair service at an additional cost for customers sending me their broken mirrors. I have successfully repaired many dozens of mirrors from my UK Customers. However, there are a couple of reasons I try and push people in to fitting the glass themselves and not send me their mirror to repair wherever possible. The first reason is something most people probably aren't even aware of! Your mirror has a lot of wiring going in to it. When you disconnect the mirror from the plug that takes all this wiring in to the car some things will no longer function. You might have already guessed that your Alarm LED will no longer flash (as it is built in to the bottom of the mirror). On some later vehicles the alarm/remote-central locking may not function with the mirror disconnected. On these vehicles, your key will no longer allow you to operate the central locking off the button. You will have to lock your car manually with the key in the lock on the Drivers door. Central locking still works off the key though. This can be a slight annoyance and also you have the worry that your car will not be alarmed while parked up. Please bear in mind this is only on later vehicles. When I remove the mirror from my 2001 E46 M3, the alarm and remote-central locking still function perfectly. You can always unplug your mirror while in situ to see whether your alarm still works before deciding on a 'Mirror Exchange' or Replacement Glass. Most Cars have Automatic Wipers and Automatic Lights fitted as standard. Neither of these will work while the mirror is not plugged in. This doesn't pose a security risk like your Alarm not functioning but can still be an annoyance just like not being able to lock/unlock off the button on the Key.
The other reason I recommend Customers changing the Glass themselves is the risk of leakage. The Reason all these mirrors actually fail is due to a Manufacturing error when the glass was sealed in the factory. There is a special Chromatic Dimming fluid sealed inside the mirror between two plates of glass. When charge is passed through this fluid it Dims. Unfortunately over time the Seal fails in one place around the mirror and allows air in to the liquid via this breach. This stops the dimming function working in either a localised spot or the whole mirror. As Air gets in to the Breach, the liquid can also leak out. It really depends where this breach is as to how quickly it can leak. If it is at the bottom of the mirror, it can leak out instantly due to gravity. If it is at the top it may never actually leak. However, if you remove your mirror and post it to me for repair then it will not be staying in it's current fitted (upright) position. Where is is shaken around in transit on it's way to me, you are increasing the risk ten-fold that the fluid will leak out. This risk is worse when the mirror has to travel further if coming from Overseas. The Fluid used in the original mirrors is very corrosive and will eat through the mirror casing, light sensors and circuit boards. In can ruin the mirror completely. This has happened before. In some cases the mirror was completely ruined and had to be thrown away. This is the reason I recommend against posting your faulty mirror anywhere if possible.
I do offer a solution for people that don't have the skills, tools, time or patience to repair themselves! I send you a complete Mirror that I have already Repaired myself with the new improved Auto-dimming Glass unit. Therefore it has been stripped, disassembled, glass replaced, then reassembled and tested. I deliver this mirror to you Recorded 1st Class and you can then do a straight swap there and then with your existing faulty unit. I can often send this mirror out on the day that you pay for it on a next day service. You could have it the day after you pay. Therefore, no time spent without your alarm not working or your remote central locking/Auto Wipers/Auto Lights not functioning. Cost of the new glass fitted is £150.00 GBP. The way I do this, is I add a £100.00 deposit to your initial £150.00 Bill. If I then receive your original faulty mirror back within 14 days and it is undamaged, I refund you your £100.00 Deposit straight back. It's that simple! This service works very well for a lot of my customers. The mirrors are very easy to remove and replace. Look at the 'GUIDES' section on my website here for how to do this.
The returned mirror must not be damaged in any way and have no fluid leaks (internal or external). The mirror you send back to me needs to be reconditioned for the next customer, so cant have anything wrong with it apart from the obvious dimming fault.
I recommend wrapping paper towels around the mirror and then cling-film to catch any fluid just incase you are unlucky enough that it does leak out in transit.