We've been discussing when we want to ask the users for their payment information. We want the sign up process to be as fast and painless as possible, so we might give the first article away for free, and only ask for payment information if the reader decides to buy a second article.
Once a user has saved a payment method to their profile, they won't be bothered about that aspect ever again when buying articles, we just charge them once at the end of the month for all the articles they've purchased that month.