I received this promo code from a startup event recently. It allows you to order one free tshirt per account. However, you can order several using the same postage address and credit card (used for verification) if you have more than one Canva account.
The offer is valid throughout December. No other terms/conditions are displayed or published anywhere from what I can see so far.
A friend received his free tshirt today after a few days shipping and the quality seems to be pretty decent.