I sell on ebay.com even though i am registered with ebay.ca - i'm not really sure what the difference is, since items show up on both anyway (and i always sell in US funds.)
With Paypal, you can have a personal account, which lets you receive money from others who have funds in their Paypal account for free. You cannot receive money if they are using a credit card. If you upgrade to a Premier account, you will be charged about 3% of the amount for EVERY transaction you receive, regardless of whether the buyer is using credit card or Paypal balance to pay. (Sending money is free, of course.)
Many people have two Paypal accounts, and this is actually a smart idea. If the buyer wants to pay by Paypal balance, they are asked to send to the first account (a personal one.) If the buyer pays by credit card, they are asked to send to the second one (a Premier one.) You can then always transfer funds from your Premier account to your Personal account, and thus avoid being charged any 3% service fees.
However - in order to have two accounts, you need to have two separate email addresses and two separate credit cards registered. Also, Paypal only accepts electronic fund transfers from US bank accounts, so a Canadian bank US$ account won't work.
Xpresspost is good - you can also offer Expedited parcel to the US for packages, the cost is lower, but they still get a tracking number. It's not guaranteed delivery, however. Oh, and here's a handy tip: if you use Xpresspost, always check on the date the package was delivered. If it's past the guaranteed delivery date, Canada Post will refund the shipping cost to you. I have had this happen a few times, and I have usually refunded the shipping to the buyer as well.