ok, the reason is the size of the shirt (XXL).
Due to the size it does not fit a regular mail box opening and therefor we have to pay more for shipping costs which is reflected in the higher credit amount.
The Dutch postal service is really strict when it comes to mail fitting through a mailbox opening
I even have a cardboard thingy that simulates how big a standard dutch mail box opening is so I can see if a package fits or not.
By the way, the higher shipping costs have always been there for the XXL size.