Vasanth & Co — gst wrongly charged 28% instead of 18%

Address: 560030

I had purchased sound bar on 23rd july with rs.19500/-, but gst charged me 28% instead of 18%, so i asked them to refund for excess paid, but they couldn't refunded and they are saying they will adjust the amount by reprint (Changing the product value and gst equal to rs.19500/-), but i didn't agreed, because tax only changed to product value for which they are not accepted and also the product have not delivered to me.