FitnessRepairParts.com Refused to give me a full refund for a defective part wentzville, Missouri

FitnessRepairParts.com sent a defective circuit board for my treadmill.  The part costs $550.00. I had originally hired a professional at $85 to diagnosis the problem with my treadmill.  He recommended that I order this part.  I received the items and paid the professional hire another $85 to install the treadmill circuit board. He said it was defective and told me to call Fitness Repair Part to ask for a refund. I called and was informed that they have a policy regarding defective items which is that they do not give full refunds for defective items.  They will send a replacement which I explained that I do not want.  At this point I do trust them to send a working part and I have to pay an additional $85 to have it tested. I wanted a full refund which they denied due to their policy. My problem is that I wonder why they have this policy - are they sending out numerous defective items and taking advantage of people? I believe it is suspicious that I have only ordered one item from them and it was defective. The restocking fee is 30% of the original fee is returned which also seems like a ripoff. It really seems like a scam and it seems odd that such a small company would not work with me for customer service reasons. I feel like I have been taken advantage of and that they should be investigated to see how many defective parts they are sending out to trusting individuals.