SHEIN & BUFFALO logistics

Hi who can I raised my dissatisfaction with? Regarding the hidden cost between BUFFALO and SHEIN, because I order from SHEIN based on a promise that my order was free as it was above a certain threshold, then all of a sudden I must pay you guys 40% of my order. This is nowhere mentioned, it’s daylight ******* & unfair. Where do I voice out my disgust about the matter?