The online analogue of the real-world grocery practice where CPG brands pay retailers for premium physical shelf placement ("slotting fees"). Instacart digitized this as Instacart Ads: brands pay for premium placement in search/browse results, the digital equivalent of shelf position.
The general pattern is broader than Instacart: look for an existing, well-understood offline monetization mechanism in your industry and ask whether it can be digitized as an ads or placement product when your core transaction fee is capped or otherwise can't be raised.
This is the specific monetization mechanism made possible by adding the Four-Sided Marketplace (Instacart's CPG Advertiser Side) advertiser side.