1. Go to Marketing > Email > Billing.
2. Move the Estimate your price slider to your expected monthly volume so the right tier is highlighted.
3. Click Sign up on the plan card you want (your current plan shows a disabled ✓ Current Plan button instead).
4. A confirmation modal Subscribe to this plan? explains that Your card on file will be charged. You can cancel anytime from your billing settings. Click Confirm & Pay.
5. The card already stored on your Sprwt billing account is charged as a monthly subscription (an existing email subscription is replaced) and the new plan becomes active immediately.

If the charge fails you see the payment error; if no card is on file the request cannot be processed (Unable to process the charge). After signing up, the top of the Billing page shows an Active Plan card with the plan name, billing cycle and this month's usage.