Within 5 months twice seams risen response by the seller "... We can not accept your claim"

When I received the bag for the first time you met my expectations. However, after a short time the seams had opened. My first complaint was handled quickly and I was given a new shell. However, after a few months the stitches went on again and this time even dissolved components from the substance. On my complaint the seller wrote "Even your first return, we will only accept a gesture of goodwill. We give no guarantee on Verschleissware like bags and bumpers. We can not accept more complaint from you."
What I did not like in addition to the quality of the customer service by the seller. He may not be the manufacturer, it could at least express regret as a seller. I put a decent amount for the Flipcase. It was unfortunately a wrong purchase.