Hi,
I have at least partially the same problem. Steps to reproduce:
1. Storefront as theme and bank transfer as default payment method
2. Add item to shopping basket and go to check out page.
3. As bank payment is default, customer gets to fill in name and address with standard WooCommerce form. Shipping cost is calculated based on zip code per (new) standard WooCommerce functionality.
4. If payment method is changed to Payson, shipping cost is still present, but not recalculated based on zip used in Payson iframe
5. As an admin, I change to Payson being the default payment method.
6. Start a new shopping session, add item to cart and go to check-out page.
7. Actual: No shipping cost is ever added. Customer can complete payment without paying for shipping.
7. Expected: Shipping cost to be calculated based on zip using standard WooCommerce functionality.
This is a blocker bug for me.