Defective product

Last year November 2018, I purchased a HP Laptop and printer for my office from a gateway outlet. Since day one we had a problem with internet connection where this laptop would be so weak with only one bar connectivity. I eventually put it back in the cabinet because it was just a spare machine. A few months ago I went back to the shop to inform them about issue. I was told, sorry we can't help you for free, we will charge you R400 without guaranteeing that it will get fixed. I just left and asked my company IT guy to come check it out. He ran all the tests, updates, trouble shootings etc and contacted HP, and this seems to be a generic problem with these machines. So it is clear to me that I was deliberately sold a defective product. These were my IT guy's findings: This machine is still under warranty for the next 50 days. So my concern and problem is why is this outlet imposing charges on a defective product?