Firsttech bad bad bad

I purchased an inverter from Firsttech. The unit started to run out of battery life after just 1 hour. I have another two of the same brand, not purchased from them, which have no problems. Have been trying to get FT to collect the faulty unit for over two weeks. Always a story. I even delivered the unit to their door but they don’t allow drop off. They sent a courier to collect who informed me he can’t take it as it’s too heavy for him on his own. Then FT asked if I still have the packaging, which I don’t, and have not heard from them since. My question is this. I had the unit at your door, why not just accept it ? Instead you insist a courier must collect, at my cost, yet they refuse ????