NEW YORK — Ivy Wu did not immediately need the navy cocktail dress that she ordered the other day. But when a representative from Shoptiques, an e-commerce site, arrived at her Midtown office with the dress only hours after Wu, 26, had placed her order, ‘‘I was really impressed that it was here,’’ she said.

This holiday season, same-day shipping has replaced free shipping as the new must-have promotion. It’s logistically complicated and money-losing — and may not even be a service consumers want or need, analysts say. But retailers from Walmart to small shops like Shoptiques are willing to take the risk. Even the Postal Service has introduced a same-day option for retailers.