Disney Store takes up to 75% off a selection of apparel, toys, home items, and accessories. (Prices are as marked.) Shipping starts at $5.95, but orders of $75 or more receive free shipping via coupon code "SHIPMAGIC". A couple of best bets, both with $5.95 for shipping:
Walmart offers this Disney Boys' Mickey Mouse Graphic T-Shirt for $5.88. Opt for in-store pickup to avoid the $4.97 shipping charge. That's the lowest total price we could find for a similar Mickey Mouse shirt by $1. It's available in size 5T only.