A best bet is the
Hanes Men's Tagless Crew T-Shirt 3-Pack in White for $8.50. Buy three (nine shirts total) for $25.50, which fall to $15.50 in-cart. With free shipping, that's $1.72 per shirt and the best deal we've seen on these T-shirts. (10 of these T-shirts start at $28 elsewhere.) They're available in sizes S to XXL. Update: They've since increased in price to $10.50 per 3-pack.
Note that for other purchases, Dollar General also takes 10% off with no minimum purchase required, also with free shipping. (The discount is automatically applied in-cart.)