Once a customer has made a single purchase, whether a product, membership or bundle, their card will be on file with the payment method. This card-on-file can be used any number of times going forward as long as their credit card is still valid. This article may also be helpful: Using 1-Click  Purchase Links

NOTE: 1-click upsells will only work if you're using a payment method that supports card-on-file functionality, you can review a list in the Payment Methods Overview article.