Find the answer to your question
Why is the CheckoutStatus on my transaction not getting updated to SellerResponded when I send an invoice for the Order that contains the transaction?
What is going on?
The Sandbox has a major product limitation with regards to Order processing.
The result of this limitation is that any kind of update made to an Order will NOT result in the transactions within that Order getting updated.
In the case of a seller sending an invoice to the buyer once the buyer has requested a total on an Order,
the Transaction.Status.CheckoutStatus field will not get updated with a value of SellerResponded.
The value will remain BuyerRequestsTotal for each of the transactions within that order.