Trendyol Order Return and Refund Hassle

I ordered a 900 ml thinner from the T***** Y*** Decoration store on Trendyol, adding a 320 ml product to qualify for free shipping. The total was 245 TL. When the products arrived, the seller sent a 220 ml product instead of the 320 ml one to qualify for free shipping.
I wrote to Trendyol assistance on August 1, and they said I would receive a response within 3 business days. When the response was delayed, I initiated a return. I had no idea they would deduct the return shipping fee from me. I returned a product worth 70 TL, but I only received 30 TL back. They claimed I could not benefit from free shipping.
The seller, who sent a product that was 1/3 less than what I ordered, is not held accountable, and Trendyol is not addressing my concerns. As customers who shop carefully, we are the ones being harmed. The product that was supposed to cost 175 TL ended up costing me 205 TL. This is unfair to customers who expect free shipping. My business has also been disrupted. Is this justice? If this is how it is, I will choose not to shop at Trendyol again, even if it means finding cheaper alternatives.