- Depends on how you initiated the subscription.
- The exact description of the charge seen on your billing statement is also helpful to determine who is billing you.
Since you posted in the Apple Pay forum, it may be that this is being billed to a card in your Wallet directly from the merchant. If that is the case you would cancel or request a refund just like you would when not using Apple Pay. First contact the merchant and if unresolved it would require disputing the transaction through the bank of the card that was used.
It is important to understand what Apple Pay actually is, it does not mean Apple is managing that transaction or is the one transferring the funds. All it means is that your device is securely sending a unique token that your bank gave you when the card was added, so the merchant does not have your actual credit card number. The advantage to that is if there is a breach on the merchant servers, your account number is not exposed and available on the dark web. The transaction proceeds directly between the merchant, PNO (VISA/MasterCard, etc), and the bank of the issued card without any involvement from Apple.
If you mean that this is an in-app purchase where your Apple Account is being billed, then when you cancelled is important. On the authorization screen you accepted by double clicking the side button, it states to cancel at least a day (24 hours) before the renewal to avoid a charge. You did say that you cancelled before the renewal, but it is unclear if you meant a full day before renewal as agreed too. Requesting a refund from a transaction managed by Apple is done by using the link below. In most cases all transactions in the App Store are final due to those explicit actions required to accept the terms, but at times Apple may provide a compassionate refund. It just should never be expected.
Request a refund for apps or content that you bought from Apple - Apple Support