How do Stripe's processing fees work when it comes to security deposits?

To our knowledge, Stripe does not charge hosts for holding a charge on the guest's credit card. Processing fees apply when payment is "captured" and moved to their account. 

Visit this article from Stripe for more details: https://stripe.com/en-ca/pricing#pricing-details

A credit card authorization can fail for many reasons. Learn about them by reading this informative article written by Stripe: Decline codes