Ah maybe thats the problem, but I believe the module should work a bit differently then. I have 10 days there.
My situation: I have two order status, which mean the order were shipped ("shipped" & "order finished"). The process is always the same. Every order is first "shipped" once its shipped & paid, its "finished". Now when an order is 9 days on "shipped" and then changed to "finished", it means, that the email will take 19 days, cause its starting the counting again.
In my opinion the module should also check for ps_order_history as many people just want something like: 10 days after I shipped the order, the email should go out. In my case I don't see any possibility to achieve this.
How do you think about it?