Why so I need to pay these extra charges?

About a month ago, I ordered something with a value or $27 on an online clothing store in china (I live in Canada). I used my card to pay for the item and shipping fees right when I ordered and so I believed that was it and I got my packages 3 days later. About a month later, meaning now, i recieved a mail from FedEx telling me I owe them $17 for customs duty, hst, ancillary service fees, and an advancement fee of $10? What I don't understand is that I paid for everything before the I recieved my item and so why are they mailing me a month later that I owe fees? Can someone explain to me why I have to pay so much on a item that was only $27? I paid for the shipping which was $10.