How can I let players buy things inside my Unreal Engine mobile game?
unreal ships with mobile iap built in through the online subsystems, no third party sdk needed for standard consumables:
enable OnlineSubsystemGooglePlay for android and OnlineSubsystemIOS for ios in the plugins, and set them as the default services for their platforms in default engine ini.
define the products on the store side first: product ids in google play console and app store connect, since the engine queries the store rather than storing anything locally.
in blueprint the flow is: query the store for the offers (so prices come from the store, not hardcoded), show your own ui with the returned prices, then the purchase checkout node with the offer id. receipt querying and restore purchases cover reinstalls, and consumables need to be finished on the store side after fulfillment.
test with the sandbox paths: license testers on google play and sandbox accounts on ios, live store purchases will not fire in development builds.
keep the grant logic server side if the purchases unlock anything persistent, or at minimum re-verify receipts on startup, since a client only purchase check is trivially spoofed on android.