EBay is a venue.
Like the mall.
You buy your groceries at Loblaw's, your lingerie at Victoria's Secret, toys from Toys'r'us, that cute purse from The Bay, and shoes at Towne Shoes.
And just like at the mall, you pay each vendor when you buy.
Not all together as you leave, like WalMart.
However.
If the problem is that the vendor is not sending you an invoice, check that she actually does ship to Canada.
If she does not, the transaction will fail. And a good thing too, because if she decides she will allow you to buy, she could charge whatever she thinks is fair for shipping and eBay will back her. No matter how over the top it is. And tracked shipping from the USA can start around $25.
Or did you buy on Friday? Some vendors do not work on weekends.