Banana Republic at Costco is one of the best-kept fashion secrets in the warehouse. Premium fabrics, modern cuts, and prices that make you question everything you’ve paid at the mall. The runaway hit is the Banana Republic Men’s V-Neck Tee 3-pack ($21.99, 644 reviews). That’s under $7.50 per shirt for soft, fitted tees that rival $30 retail versions. For women,…