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CHICAGO (CBS) – ‘Tis the season for shipping out holiday gifts, but how do you know if your packages are being weighed correctly? And what differences are there in shipping costs? CBS 2’s Dorothy Tucker went out to get some answers.

We set out to make sure our most popular shippers were weighing our packages accurately. We weighed a small brown package on a scale checked out by Chicago inspectors. It came in at 5.6 pounds.

Then, we took it to three FedEx locations and three UPS stores. Every place we checked, it weighed the same as it did at CBS.

What was surprising was that the companies round up to the next pound to determine how much to charge customers.

“So if it’s 5.1 pounds, we would measure that as 6 pounds,” said Greg Stuhrenberg, a store manager with FedEx. “We do write 6 pounds, so we do try to communicate that as part of the process.”

UPS told us the same thing. But the rounding up policy is hard to find in writing.

We couldn’t find it at any of the FedEx or UPS stores, or in their brochures. After searching, we did find something on their websites.

“I didn’t know they were doing that,” said Varun Sharma, FedEx customer.

At two of the FedEx stores, we were quoted $14 for a 6-pound package. Online, the price came out to $13.30.

At UPS, the online price was $14.52 and about the same in the store.

There’s nothing illegal about rounding up, but some customers just don’t like it.

“That’s not nice, especially with people and the economy how it is, and people don’t have money right now,” said Macray Huff, FedEx customer. “They need to round down.”

We also took the package to four U.S. post offices where a spokesperson says all the scales in the system should record the same weight.

At three of them they did. But we discovered a minor problem at the office on Clark.

At the automated center, the package weighed 5 pounds, 9.8 ounces which equals our weight. But, when we took it to the counter, the clerk said it weighed 5 pounds, 9.5 ounces; just a few fractions of an ounce off.

“That’s surprising. I don’t know why that would be,” said Mark Reynolds, a spokesperson for United States Postal Service. “I would have to take a look at that and see what is actually happening there.”

The post office ended up being the cheapest option, at $12.02.

CBS 2 suggests that you weigh your package on the bathroom scale. Then look up the cost on the company’s website. That way you’ll have an idea of what you’re going to be charged before you take it in.