If it's a seller specific coupon, it is usually sponsored by that seller. This is likely in cases where the platform is mature and relies on cash flow to sustain. Sellers can't piggyback on free marketing from the platform and need to spend money vying for attention on the platform.
Although in either case, because there's a shared interest, it could be a joint sponsorship with both platform and seller sponsoring a certain portion of the discount based on agreed terms which can vary from coupon to coupon.
I would further suggest that the sale price of the product is increased by a said amount.
Or we could look it up... (its 30%) To do this I asked Bing "how much does it cost restaurants on uber eats"
https://www.ridester.com/uber-eats-fees/#:~:text=Uber%20Eats....
https://entrecourier.com/delivery/gig-delivery-platforms/ube...
https://twitter.com/grubhub/status/611320394256109568?lang=e...