USPS shipping is an option for items listed on eBay.com but not eBay.ca. eBay.com does not offer sellers the option of listing Canada Post as a shipping method.
Is the currency of the listing US dollars? If so, that's a sign that it's been listed on the .com site.
Your seller may sell mostly to the US and has set up his or her listings to make them more attractive to US buyers. In addition, your seller may have found a way to purchase postage from USPS and ship his/her items from a US location.
I wouldn't worry about it. The main thing in the long run is whether or not you get your item.