[w3c/payment-request] Ship Mode Update for PO BOX (#888)

Here's an issue I am running into with Payment Request in regards to PO Boxes and Ship Modes. 

We have 3 Standard Ship Modes that user can select from: 
**Overnight - $30**
**Express - $18**
**Standard - Free**

We can calculate shipping modes based on State and update accordingly. 

User in this case selects Standard Ship Mode. 

For users with a PO Box as shipping address, we only offer one ship mode which is unique to PO Box. 

**Unique Shipping - $25**

The change here would affect total cost of product with shipping and would need to notify user that ship mode has been updated to reflect PO Box address. 

We have limited ship modes for users with PO BOX as shipping address. But, since addressLine is not exposed until user commits to payment, how would Payment Request handle this case?

Would we use the retry method and throw an error and advise user to select the updated ship mode? 
https://www.w3.org/TR/payment-request/#retry-method


cc @rsolomakhin 

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https://github.com/w3c/payment-request/issues/888