Generate two random numbers, save them in your database, auth those amounts on the card, void the auths, and check if the amounts the customer provides match wh

Generate two random numbers, save them in your database, auth those amounts on the card, void the auths, and check if the amounts the customer provides match what you saved. You can do this with nearly any payment processor. I would be surprised if there's anyone that wraps this little interaction up as a service.