UPS and FedEx will soon change their pricing mechanism for ground shipments, so the Postal Service is taking out advertisements to remind businesses about its affordable alternatives.

In late December and early January, UPS and FedEx plan to expand their use of dimensional weight pricing. This means the companies will charge customers by a package’s actual weight or its dimensional weight — the amount of space it takes up — whichever is greater.

In some cases, shipping costs could rise 30 percent, Forbes reported last week.

USPS is responding by running ads in business magazines and other publications that promote the Postal Service’s “broad range of efficient and economical options.”

One ad begins by asking, “How much does a 1-pound box weigh?” The answer: “11 pounds, if you use the wrong shipping company.”

The ad campaign fits with the Postal Service’s aggressive push to grow its package business, including adjusting some shipping prices to attract more business customers and investing in new package sorting equipment and other upgrades.