The pricing doesn't look that special to me. Similar to Full Throttle or World of Accesories.
Fitting your own brake pads should not impact on the warranty, unless of course the brake pads damage something like the discs.
The dealership I use will fit for you, but the saving wouldn't be that big as the dealership will also sell Ferodos instead of OEM to you at your request. My wife's LC just had rears done as part of the service and the price was around R500 for the pads.
If you know what you are doing you should be able to do the rear pads in 5 or so minutes.