Your charity does. Donations are charged directly on your charity's own Stripe account, with your charity as the merchant of record, and Stripe pays them out to your bank. Donors doesn't hold donation money. Stripe's processing fees are charged by Stripe on your account. To see payouts, click Stripe dashboard on the Stripe card in Organization Settings → Integrations.
If your donation form has Offer to cover processing fees turned on, donors can choose to add 3% to their gift.