Tradeshift wishes to allow both parties to maintain a clear overview of all their transactions, and to ensure visibility and instant communication, right on the invoice itself. This is why your customers receive email notifications when a document changes state. 

If your customers do not want to receive emails every time a document changes state, it is possible for them to turn off these email notifications by:

1.) Changing their own notification settings as shown here (if they have accounts on Tradeshift).

2.) Disabling document state notifications right from the emails they receive (if they do not have accounts on Tradeshift). The screenshot below shows how a state change notification email looks like. On the blue part there is a link that allows your customer to disable future status notifications.