Thanks for the guidance! The issue was indeed related to a malformed promotional offer signature. I was generating the “classic” promotional offer signature (string + ES256) instead of the Promotional Offer V2 JWS that StoreKit expects. Following the recommendation to use the App Store Server Library as a reference, I updated my backend implementation to generate a proper JWS (ES256) with: aud = "promotional-offer" iss = issuerId bid = bundleId iat and nonce productId and offerIdentifier (and optionally transactionId) Header including typ: "JWT" and the correct kid After generating a fully compliant V2 JWS and passing it to promotionalOffer(_:compactJWS:), the purchase works correctly in Sandbox on a real device. Thanks again for pointing me in the right direction — using the official library as a reference was the key.