We purchased a oxygen concentrator on the 4th of Aug. for $450.00 plus $70.00 shipping. We received it about a week later and it over heated after 2 hours of use. We sent it back and they texted me that it was damaged in shipping. We received another one a week later. It worked for 16 hours and started shutting off after about 3 min. of use. We did the trouble shooting with no results. We then called them and they had us try a few things that didn't work also. We sent that one back and just wanted a full refund at that point. After almost 2 weeks we got a refund of $450. but not the $70.00 shipping. When I contacted them about the shipping fees, all of the sudden it was our fault that the machines didn't work . I have never had such a terrible on-line shopping experience. In their add they gave a 30 day guarantee that they didn't honor.