Well for doing Canada to US, when you go to make a payment, you can select the currency. Then Paypal will give you two options for conversion. Either let paypal do it (the exchange rate they quote you will have a 2.5% service fee), or you can let your Credit Card handle it. I don't have my bank account linked and only use my CC with Paypal.
Check their help, it's fairly straightforward.
--Mark