Transaction is unsuccessful due to a SHA-Out issue
If you or a customer see the warning: Transaction is unsuccessful due to a SHA-Out issue
You need to check what parameters are sent back to the website (This is used the decrypt information about the transaction).
Login to the PostFinance back office and navigate to here: Configuration > Technical information > Transaction feedback.
You will find the setting under the Dynamic e-Commerce parameters section
You only want the following in the selected section: