In the Reservation Detail Panel you can see by clicking the Stripe button if you already have a Stripe card token from this customer. You can use this later to charge the final rental total or security deposit. Or add a new card in the same screen: Adding Stripe cards for a reservation

With this information saved in BRM you can take a new transaction by clicking Add Transaction button in the RDP or advance the reservation status and for the popup asking to charge for an open balance: Adding Transactions