Having worked in offices for a fair few years, I understand each package needs to be labeled individually, but to post one at a time, seems rather a poor use of time.
Surely as you check the size, place in postage bag, add label, next t-shirt n label etc, then go to postal office with a couple bags with packages, go to business counter, get each weighed n stamped (if you don't have a franking machine in the office) and in under three hours, 30 t-shirts posted.
I've done many jobs like this, and a prime reason for mail merge on computer. Search all level 7+ users, and merge details into Excel to show details and sizes... Good old print n tick (off each person as sent/packaged).
I'm not being ungrateful, but small (even a large is the same method) runs like this, shouldn't/needn't be an issue for such a large company as you.
I can think like this n how to do at 6am after 11 years of not working in an office.... Imagine how good I was when still doing this job‽