1) Customers can select another variant of the same product. You can see a demo of this here:
2) if you are on our Premium Plan or higher, you can click "Exchange Order" inside our dashboard, and the exchange order with all details is created automatically. Click here to see a demo of the feature:
We create "Free" Exchange Orders only, so we don't automatically charge the customer for any price difference. Since variants often have the same price, this is usually not an issue.