In many respects, this has been the best of times for U.S. exporters. Merchandise exports in the first six months of the year were up a stunning 20 percent compared with the same period in 2007, rising from $550 billion to $662 billion, according to Census Bureau figures.

But it hasn’t been a summer of fun for many of them. Instead, it’s been a summer of frustration for many exporters, freight forwarders and non-vessel-operating common carriers.

Take David Masafi, an NVO in New York City. On a recent sunny afternoon, he was working the phones trying to arrange a shipment of a customer’s household goods from Buffalo, N.Y., to Switzerland.

There was only one problem. He could not get an affordable container. Freight costs have shot up in the past year thanks to rising bunker fuel costs. As a result, the booking failed.

“I lost that $6,000,” said Masafi, barely able to hide his frustration, referring to how much he would have made on the shipment. “We have canceled several jobs because we cannot get a container.”

The gregarious 39-year-old Israeli-American has worked in cargo shipping for nearly 20 years. He has never seen freight rates this high. For instance, the base rate for shipping a 20-foot container from New York to the U.K. is about $1,300, up from $600 a year ago. Additional general rate increases could push that figure above $1,500. And that doesn’t include the fuel surcharges, which now range as high as $500 for a 20-foot box.

In recent years, Masafi and his wife, Einat, have been growing their shipping and moving business. Einat, a bubbly woman in her mid-30s, handles marketing for their company, New York International Moving & Shipping, which is registered as an NVO.

Masafi said business was great until this year when oil prices doubled to more than $140 a barrel, pushing up the prices of all types of transportation fuels — from diesel used by truckers, which now retails at $4.50 a gallon, to jet and bunker fuels — and denting his profit margin. Oil prices have since come down to less than $120 a barrel, but that’s still nearly double year-ago levels.

“The problem starts with fuel and ends with fuel,” he said.

He is not alone. All shippers are suffering from rising freight costs. Many have seen a dramatic decline in their business because fuel surcharges and other associated costs have gone up by as much as 50 percent, scaring away many potential customers who already are hurt by a slumping economy.

“We are seeing a 25 percent reduction in profit margin and our operating cost is up 30 percent,” said Tony Cavaliere, who owns Shipping Overseas Specialists Inc., a freight forwarding company in Newark, N.J.

Meanwhile, U.S. exporters may be losing some of their competitive edge because of the dollar’s recent strengthening. After reaching its low point of $1.59 against the euro on July 14, it bounced back to $1.48 as of Aug. 14.

A new study by economists Jeff Rubin and Benjamin Tal of CIBC World Markets Inc. demonstrates the impact of rising oil prices on trade. Eight years ago, when oil — particularly U.S. light sweet crude — was trading at nearly $20 per barrel, shipping costs were the equivalent of a 3 percent U.S. tariff rate. When oil reached nearly $150 per barrel, the tariff equivalent rate was 11 percent, nearly the equivalent of the rates seen in the 1970s, according to the study.