it will be 7% plus 6% GST plus 7% PST, so that would be 20% total.
On a $500 purchase you are looking at $100 in taxes.
As long as you don't bring your old tires back with you, then no one will know.
If you want to keep your old tires and bring them back, then I would recommend declaring the purchase. Otherwise, I had a friend of mine who bought $3,500 for some rims and tires and he did it without declaring and still brought back his old rims and tires.
what he did was tell the shop in advance that he would buy the new rims and tires if he could store the old rims at the shop for a week or two. And what he did was take out the spare tire, and replace it with his old rim. He made 3 additional trips to bellingham and brought back the old rims that way.
Now his rims where spanking new chrome rims, and he got away with it. So, with just tires, you should just be fine. Just have a story ready in the rare and i mean rare chance they ask you about it. even if you get caught, deny it, they can't prove anything.