While one-time payments are a great feature idea, it's important we stay true to what Patreon is all about: Building a membership platform for creators.
For now, the best way for a patron subscribe to a creator with a one-time payment is to become a patron then simply cancel their membership after they have been successfully billed. You'll know when you have been billed because you'll receive an email confirmation stating your payment has successfully processed.
Additionally, you can check your billing history to see when you were billed and if it has successfully processed.