I would avoid UPS at all costs and I believe a gift declaration is only good for a $60 exemption. Using UPS (and other couriers to a lesser extent) will get customs brokerage fees levied upon you.
In the past I have had tires shipped to UPS depots or shipping centers located in Niagara Falls or Buffalo. You can clear customs yourself but you'll have to produce a receipt yourself. This has the added benefit as some tire companies might offer free shipping anywhere in the US as opposed to paying high amounts for shipping right to your Canadian address. Additionally, you will save yourself some custom brokerage fees.
Also if your current wheels have worn tires on them (the ones you're going to replace), just hit up a tire store stateside after receiving your tires and install them there. The weigh scales will not pick up on the weight differential, between egress and ingress, because there won't be any.
Beware crossing at the Peace Bridge (@Fort Erie), they will go through your stuff because they have some agenda. The Rainbow Bridge (@Niagara Falls) is much more lax.
And oh btw, if you haven't driven to the US in a while, remember to bring your passport now.