This study examines how commission fee pricing reshapes multi-sided user-generated content (UGC) platforms by analyzing a natural experiment where a major platform eliminated fees for goods-selling ad links in sponsored contents. Using various market-level difference-in-differences models, we reveal three key findings: (1) Advertisers reallocated 13\% of placements toward fee-exempt links, demonstrating notable price sensitivity; (2) Platform revenue declined as substitutions outweighed demand expansion; and (3) The fee exemption triggered a redistribution on the creators' side, with contents for treated advertisers gaining 3 additional interactions per 100 views, occurring primarily from top creators. Our work advances information systems research by providing the first causal evidence on how commission fee changes reconfigure multi-sided markets—spanning advertisers, creators, and platform revenues. The findings highlight critical trade-offs: while fee structures steer advertiser behavior, they may inadvertently cannibalize revenue and amplify redistribution among participants in this multi-sided ecosystem. These insights inform platform governance by demonstrating the need to balance participation with profitability and model cross-sided effects before implementing pricing changes.