It doesn't matter if you pay in advance, technically. While I realize a Tivo box isn't the same as a puppy, in the eyes of the law, they are, so here's an example.
I bought 2 Tivos on eBay. I had them shipped to my PO Box in NY state. I went down for the day and picked them up. When I returned to Canada, even though I had already paid for them, I still had to declare them AND pay duty on them as they were acquired in the U.S. and I was bringing them back to Canada. Had I been down for the weekend, my allowance for duty free goods would have covered them, but since I was gone less than 24 hours, I wasn't allowed anything duty free. Mind you, on $240 worth of goods, the duty was $30 odd.
Hope that clears things up. The only way I see it being an issue (hence the reason I say have a cheap receipt written) is if you get a smart border guard, they may notice that their shots were recent and were given in Canada, leading them to ask questions. If I were you, I would declare the puppy, say you paid $50 for him/her, you saw him/her on vacation, and make that the end of it. If you're gone at least a weekend (48 hours) you would be allowed to bring back $400 (I think) so it would be a moot point.
Tara
