We do not support the standard state and country picklist options being enabled for Salesforce.  This is due to our data validation routines having different formats for the states in different countries.

The state/country picklists if enabled become restricted picklists and this can cause issues with the Matching / De-duplication rules.

To get validate data on the checkout forms, see our data validation tools with Kleber.

https://help.payments2us.com/m/userguide/l/1419674-data-validation-overview