WASHINGTON — Expanding on the success of its line of Priority Mail Flat Rate Boxes, the U.S. Postal Service has introduced the first Express Mail Flat Rate Box.

Ideal for consumers and small businesses selling merchandise online, the Postal Service’s Express Mail Flat Rate Box takes the guess work out of shipping, providing overnight delivery — up to 70 pounds — almost anywhere in the country for $39.95.

“With the Express Mail Flat Rate Box, there’s no need to use a zone chart to calculate prices. If it fits, it ships,” said Gary Reblin, vice president, Domestic Products. “The Express Mail Flat Rate Box offers convenience, ease of use, free packaging and a flat-out value price.”

Express Mail Flat Rate Boxes also can be used for international shipments with a maximum weight of 20 pounds. The boxes are priced at $59.95 to Canada and $74.95 for all other countries that accept Express Mail International.

The Express Mail Flat Rate Box rounds out the Express Mail Flat Rate product line, which includes an Express Mail Flat Rate Envelope and an Express Mail Flat Rate Legal Envelope, both retailing for $18.95.

The same size as the medium Priority Mail Flat Rate Box, the Express Mail Flat Rate Box is available in a top-loading or side-loading design. The boxes can be ordered online at usps.com and also are available at select Post Offices.

Express Mail service offers a money-back guarantee (some restrictions apply) and overnight delivery to most U.S. addresses, including PO Boxes. Express Mail is delivered 365 days a year with no Saturday delivery charges, residential surcharges or fuel surcharges. Sunday and holiday delivery is available to many ZIP codes for a modest premium.