Adding second shipping option in Magento & adding sales tax for US and Canada

I need someone to help me to add another option in the shipping method in Magento (currently I have a flat rate setup for shipping in US only). The shipments to Canada can't be flat rate, it depends on the amount of $. For example, if the customer orders $200 worth of merchandise, the shipping cost is $25. However, if the customer's order is $400, the cost of shipping will be $40.

Also, i need someone that can help me to add the extension for sales tax in US and for Canada.

Thanks!!