Why those fuel surcharges will never go away

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A UPS truck in San Francisco

The recent plunge in gasoline prices has brought big savings for shipping companies that use a lot of fuel.

Then why are they still charging their customers a fuel surcharge? (Tweet This)

This week, this CNBC.com reader is wondering why he still has to pay UPS a little extra to fuel up their trucks and planes—now that the price of diesel and jet fuel has fallen by roughly a third from last year's peak.

I just a paid a fuel surcharge to ship a package UPS to my son in Los Angeles. I guess I could live with it when gas was over $4.00 a gallon, but how can companies justify retaining it when gas is $2.50?
- Peter B., White Plains, NY

Hang in there, Peter. Next time you have to ship your son a package, you'll likely see that surcharge drop a bit. But you won't see it go away.

Shippers like UPS and FedEx began including such surcharges years ago to cope with the volatile price of one of their biggest single costs. By itemizing the cost of fuel, they can keep their "base" rate relatively stable. That base rate typically changes only once a year, according to UPS spokesman Andy McGowan.

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But the fuel surcharge changes every month. And there's a two-month lag between changes in the price of fuel and the surcharge you pay—so this month's surcharge reflects the price of fuel in February. If fuel prices keep falling this month, you'll see those savings in June.

The good news is that fuel surcharges have been falling, according to BirdDog Solutions, an Atlanta-based shipping consultant. According to their data, surcharges dropped sharply this year as lower fuel prices worked their way through the system.

That doesn't mean your overall cost of shipping that jacket your son left behind the last time he visited is going down. As the economy has recovered, and demand for shipping has tightened, shippers have been gradually raising rates. For large commercial customers, those rates are set by negotiated contracts. As those contracts roll over, shippers are looking to boost prices.

For customers like you and me, UPS also breaks out other fees, according to the company's website, including pickup charges, "return services" charges, "international extended area" charges, "residential" surcharges and surcharges for large packages. Those charges may also change as the price of fuel goes up and down.

Though fuel surcharges are falling, UPS and FedEx recently rolled out a big boost in the cost of smaller shipments with something called dimension weight" pricing—in response to the flood of small packages from online retailers like Amazon that are mostly air.

As my colleague Heesun Wee skillfully explained earlier this year, when you order a pair of shoelaces and it comes in a box big enough for a pair of shoes, that packaging takes up valuable space for the shipper. Until recently, though, the cost of shipping the shoelaces was much less than the shoes because rates were set mostly based on weight.

Read MoreHigher shipping rates and the Amazon effect

Now, the cost of shipping air has gone up. Under the new pricing scheme, the dimensions of the packaging matter a lot more.

For example, if you son leaves behind his brand new pair of New Balance Fresh Foam running shoes next time he visits and you send them back in the original 13x8x5-inch box, you'll be charged for a "dimensional weight" of 4 pounds - not the actual 2.5 pounds the box weighs.

If you want to do the math, multiply the container's length times weight times height (in inches) and divide by 139. If that number is bigger than the actual weight (in pounds) you'll pay the higher amount. FedEx has a handy calculator to figure this for you.

Or you can just wait until your son comes for another visit.

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