People that receive care packages in the mail usually end up having to pay duty.
I have asked my family not to send us anything unless we ask for it. I hate paying duty for a couple of colouring books and pens, a small purse and costume jewelry that the grandparents package up just to say "I love you."
In large part the duty charged is based on what the package cost to ship, any insurance paid, the amount of value written on the declaration form and then, if the post master is suspicious of the declared value or the item enclosed, the package is opened and the goods are assessed.
If you are charged a lot for a package that hasn't been opened, you can open the package with the post master, show him what is in there, and sometimes he can reassess the value down. ie. If you know you have 2nd hand clothes or a tiny paper gasket etc he may not see any true value in the item and waive charges.
But a large factor in all of this is.....Attitude. If you are nice and gracious you will fare much better than if you whine or get indignant.
And yes, it is absolutely shocking how much it costs to mail a t-shirt.