As one of its daily deals, HoboNinja offers this "Grow Up" Red Mushroom T-Shirt
for $9.99. Coupon code "IWILLSHAREHOBONINJA" cuts it to $5.99
. With $2.99
for shipping, that's the lowest total price we could find by $8 for any shirt celebrating this somewhat lackluster but essential powerup. (Let's be real, Tanooki and Frog are where it's at.) It's available in sizes S to XL.
Note that, after the above coupon, size XXL is available for an $6.59 and size 3XL for $7.19.