I found all of the cheats earlier than expected, so here they are. Soon I will put this on the Catalog Secrets page.
Before I give you the cheats I thought I would tell you about the customizable shirts. You can build you own shirt on the first page. First pick a color and then pick a pattern. Here is a picture (click to enlarge):
Pink Snorkel: Go to the second page and click on the snowman’s nose. (Click the highlighted area.)
Pink Flippers: Go to the second page and click the mountain behind the blue penguin.
Viking Helmets: Go to the third page and click on the big bubble by the fish’s tail. Open and close the pop-up window 4 times to get the blue viking helmet.
Stocking Cap: Click the Coins for Change bucket on the fourth page.
Long Johns: Click on the top of the tree on the fifth page.
Black Superhero Mask: Click the word “Work” at the Penguins at Work page.
The Dizzy: Go to the 9th page of the catalog and click on the white puffle on the right.
If you find any more cheats, please leave a comment.