Hi @mikebronner
You could perhaps send out an email with a link to a credit card form where they can enter their cc details, add/or set up a filter on any of your routes that require a subscription which steers them to that form.
Then if you are using Cashier and you mentioned said you are setting your users up to have stripe customer ids, you can simply do
Auth::user()->updateCard($stripe_token);
Jeff has a number of vids here about how to get the Stripe token.
Cheers!