You only get what you pay for!
I can understand your concern over the price but the problem is that to put this equipment in your aircraft involves more than just "fitting" the kit.
Each change to your aircraft requires an EASA approved modification this will involve a lot of paperwork.
If this company fitted the kit it would have been done "properly" with all the wiring identified at both ends and all the apropriate interconections made with other equipment all of this is very time consuming.
There will be people telling you that there are "cheaper" options but when I had to oversee a £35,000 avionic re-fit all the reputable companys quoted about the same price and the "best" quote turned out to not include all the interconections that thee reputable companys considered as part of the standard fit. The company that you don't like got the job not just on price but on quality and reputation for product support.
There are others who will promise better service at a lower cost but I think that you need to be mindfull of the old arab proverb that "cheap is not so cheap".
Please note that I have no financal conection with this company.