Be careful what you wish for.
Remember that Canadians have a duty-free allowance of only$20 (~$16USD) after which any courier (like UPS or FedEx) is required to remit applicable duty and Sales Taxes*. The couriers also charge a 'customs brokerage fee' of $25 or more.
While high value and bulky items like tires are likely to be stopped even if sent by USPS/Canada Post, less expensive and smaller items are usually ignored by CBSA officers who have intelligently come to the conclusion that it makes no sense to assess and collect duty and Sales Taxes when they will not collect enough to cover the cost of the assessment.
Sellers choose the shipping service they will use to protect thier investment.
Most American sellers want their shipments to be picked up, tracked, insured and to arrive promptly.
Each of those requirements costs money.
This is the USPS rate card-- look at the Shape and Size icon for best prices.
https://postcalc.usps.com/?country=10440
*Most import fees have higher sales tax than duty. The courier's fee is also likely to be higher than duty.
