I appreciate your attempt at lateral thinking, Gillian. But that isn’t actually what happens.
Filling the Stripe popup simply puts the relevant details into the s2Member form. It all happens client-side, and nothing is sent to Stripe at that stage. Clicking “Submit Form” sends all the details to Stripe, and then the user gets redirected.
I am afraid that Andrew has simply confused himself. It’s a classic case of someone reporting what they think they have done instead of simply saying what they have actually done (i.e. saying that payment is made instead of saying that the Stripe popup form has been completed). This is one of the biggest problems when attempting to help people on the forums because it means that such conversations are often at cross-purposes.