Ive bought things from this clothing website a million times, and never been charged duty except for now?

Duty depends mostly on where a product is made, not from whom it was purchased. Were the clothes made in a different country than others you purchased? If you want to know what the duty will be, you can look it up here: http://betterdollar.com

Duty depends on the value of the item (not the price on the invoice), which country it was made in, and what materials were used So the invoice might say $50 but if Customs feels the item would sell for $250 in Canada then they will charge you duty on $250. It happens a lot with fakes - if someone buys a fake Gucci bag online for $50 but a real Gucci bag costs $1000 and it doesn't state on the invoice that it is a replica bag, then you will be charged duty on $1000 which is the actual value of the item.