Did you try using the link from the New Release: The Rite Of Passage thread?
When I go to Teespring on my own (by typingTeespring’s address into my browser) I get “Featured Products” from Sapien and TRoP is not included. However, when I click on the link from the thread, I’m taken right to the product page, where I can put that field into my cart and even proceed to Checkout. (I didn’t try checking out because I don’t want to purchase that product but it seems to let me go through the whole process.)
I did all of this on my computer (rather than a mobile device) which might also be a factor for you.
Hope that helps.