We support one-time payments, but our overriding focus is on recurring transactions.
To charge your customer any amount through Chargify, you need to have at least 1 subscription in place for that customer. The subscription is what associates the customer with a credit card.
Here are a couple of paths you can take, depending on your needs:
FOR ONE-TIME CHARGES THAT HAPPEN ONCE, OR ONCE IN A LONG WHILE
If you're just going to charge your customer once in a long time, then you can define a product that only lasts 1 day (i.e. it expires after 1 day) and create a subscription that ties your customer to that product. The subscription dies (expires) quickly and it's done. This is most applicable if you just want someone to sign up, then pay you a set amount of money, and then the relationship is pretty much over.
See how to create a One-Time-Purchase Product.
FOR ONE-TIME CHARGES THAT HAPPEN MORE OFTEN WITH THE SAME CUSTOMER
If you will charge your customer from time to time, then it's better to define a product that has a recurring billing period of, say, 1 month, but has a base recurring fee of $0. You can then subscribe your customer to that product, and just add charges to the subscription when it's time to charge something. There are two ways to add charges to an existing subscription, as shown below:
2.a. There's an API call (as well as admin UI element) for processing an instant charge. You can use either of these to generate an immediate credit card charge on the card on file for the subscription.
See how to process an Instant One-Time Charge.
2.b. There are things called "Components" that may be very useful in your case. For instance, Metered-Usage Components are used to add charges to a subscription as your customer consumes units of something you're selling. These get added up and billed at the end of the billing period for the subscription.
Read about Components.