Customer service sucks

I am doing business with Mit Auto parts for 20 years. They rejected a claim i first got an rfc number as the claim was not loaded within 3 days of receiving the part.That is where my problem comes in .I received part on Friday and now they count Saterday and sunday also in the 3 days ? I checked the part on wednesday as company already check all parts before shipment! The part is marked as checked so why do they still send a defective part to customer after its being checked! They are quickly to throw policies in your face but if iy comes to them there are no fault in their doing!They don’t even take customers support and loyalty into consideration!!They flat out just void claim !! I know nothing will come regarding this but i just want to let them know that they tread customers like ******* as this is a good example of how they cannot see reason and make an effort to set their mistakes also correct They should not have send a defective part to me after it has been checked by their dispatch warehouse !