Can you please assist on this:
The shipping rate of one of my item is $9 and it works properly on checkout, however When I tried to purchase 2 quantities of same items it became $29 and when I tried 3 and above quantities it says "Sorry, no quotes are availble for this order at this time. I'm using WebShopApps MatrixRate – #1 Multiple Table Rates Extension.
Since that is a 3rd-party extension, the reason for the issue is related to the code from the vendor.
Have you tried contacting the module vendor?