Cheapest way to ship a t shirt to Canada from USA?

It probably makes a difference where in California you are shipping from, since the state covers so much area from north-to-south; and where in Canada, since it covers so much east-to-west. I imagine shipping from Sacramento to Vancouver would be cheaper than San Diego to Quebec.

FedEx, UPS, and the USPS have very easy shipping calculators on their websites. It would be easier for you to find out the exact fees from your postal code, to the Canadian city where it will be delivered.

1st class mail will be, without a doubt, the cheapest rate.
I have an ebay store, and several regular Canadian customers.
1st class packages can go up to 4lbs.

I ship things all the time to BC from NYS, usually as first class mail and occasionally at a package rate. There is a standard cost per ounce. I don’t think it matters where in the states you mail from. If it is an item rather than paper, I have to fill out a customs declaration slip, with a precise weight (lbs.and ounces) on the slip. That always means a trip to the post office.

Fold it and put in a tyvek 8×10 envelope.Then mail it first class,

Unless you really have to leave it in its package, take it out, and declare for customs “used clothing” so there will be no tariff/VAT applied.

I’m with @zenvelo. Tyvek, 1st class.