I paid the guy for shipping, and I am sure he will say it is a customs thing.
It is a customs thing... and that's why I no longer buy from and/or ship to Canada... it's a pain in the butt. Most likely he didn't complete the proper forms, which aren't easy for the average Joe.
I don't know what will happen if you don't pay. Maybe UPS will flag your address and stop delivering to you until you pay it!
If you want to get confused, look at
this page from UPS... just a few of the forms you need to complete for shipping.