Shipstation references the
Unique ID field and reuses data attached to the first order with that ID if it finds it again. So, if you use something like a customer's first name in that field like this:
Then if someone else has the same first name, their customer details will be attached to the order. To get around this, make sure you use a truly unique field for the Unique ID.
Shipstation requires that you use a two letter country code, so instead of United States, you'll need to send US. If all your orders are going to be shipped in the US, you can just type US in the Recipient Country Code rather than mapping the field (or whatever the correct country code is).
If you need to dynamically set the country, one option you have is using the Formatter Utilities action. You'll want to add this step before the Create Order action. Use the Lookup Table transform and map the possible fields coming from the trigger on the left hand side (ex. "United States" or "United Kingdom" and the two letter country codes on the right hand side (ex. "US" or "UK").
Then you'll want to map the output of the Formatter app to the Create Order action.
If you see Shipstation giving a long and obscure error for Create Order actions, check the Items section of a zap:
If any of the fields in the items section are filled in, then Name and SKU need to be filled in too.