Another thing that may be happening is Cakewalk isn't actually shipping these themselves. They may have a deal with the screenprinters who have their own distribution methods and policies and the shirts are being sent from there.
Being familiar with this type of cross border stuff the common reason I've heard over and over again as to why sellers in the US insist on using couriers is because Canada Post is NOTORIOUS for having packages from the US or elsewhere simply disappear for weeks/months and sometimes YEARS (yes I had that last one happen a couple times). USPS also has really good tracking for packages even for their regular post stuff but as soon as it hits the Canadian border those tracking numbers become useless because CP does not even acknowledge the USPS system.
What happens is the seller ships off an item, it gets to the border (which shows up in tracking) then it will say "in customs" or something then nothing. A week or so goes by and even a patient customer will begin to get antsy and contact the seller who assures them it was sent and gives them the tracking number (which simply shows it reached the border). Most of the time it will show up a little while later but with quite regular frequency it won't and the customer demands a refund or a replacement. The seller will oblige (begrudgingly) or they won't (which ticks off their customer) so they either have lost money in extra shipping or lost a potential return customer (who may tell their friends not to buy from them). If the seller sent another item then the customer may or may no get that one and then the other one shows up so now they have two. Then they contact the seller (if they are honest) to send it back or whatever and someone ends up having to pay for the return shipping or the seller will have to just eat the loss and say keep it (which many times is more cost effective for low cost items).
So that's just a couple scenarios but as a business person you can see how that gets REALLY tiresome and costly after a while. With the couriers they totally rape you on fees but they get the job done, provide elaborate tracking on BOTH sides of the border AND they are accountable if something goes missing or is late.
I'm in Canada so I had the other end of all this which I won't get into but I used to ALWAYS tell our US customers they had to wait at least two weeks for delivery... even if they were RIGHT across the border. Overseas customers were told they had to wait 8 weeks and even then a lot of the times that was a low estimate especially to places like China.
So what I would say is get a hold of Cakewalk and ask them to send it USPS (if they are indeed shipping it themselves) and tell them you don't care if it takes a little longer.
Another factor to consider is the "handling" part of "shipping and handling". Someone does need to get paid to pack the thing and drag it down to the post office or wherever it needs to be sent from. It does indeed cost money... just not as much as many sellers claim.
Food for thought.