The U.S. Postal Service’s (USPS) rude employees could cost the government $288.5 million in lost revenue, according to an audit by the Office of Inspector General (OIG).

A growing number of Americans believe USPS is "worse than other retailers" when it comes to customer service, the audit said. The OIG partly blamed the American Postal Workers Union for the bad service, since the union requires the USPS to select employees for retail positions through seniority and not merit.

"While the Postal Service’s goal is 90 percent customer satisfaction, we found that more than 20 percent of its customers in FY 2013 responded to the [Point of Service] POS survey that they have been treated ‘worse than other retailers’ when visiting Postal Service retail counters," the audit said. "Dissatisfied customers exist, in part, because procedures for improving customer service are not functioning as intended."

According to the survey, at least 20 percent of customers said the Postal Service is "worse than other retailers" at greeting you in a friendly manner (20 percent), being pleasant (21.5 percent), treating you with courtesy (20.5 percent), treating you as a valued customer (24.9 percent), and treating you with respect (20.3 percent).

The OIG created a word cloud of the top complaints about retail service at the Post Office, the biggest being workers are "rude."

The audit concluded that poor service could cost the agency millions. Though the agency is not taxpayer-funded and relies on postage sales, the USPS has $100 billion in unfunded liabilities.

"We estimate the Postal Service could risk $288.5 million in FY 2015 by failing to improve customer experiences at postal retail counters," the OIG said.

The OIG said the Postal Workers Union makes it more difficult to fill retail counters with friendly employees and suggested more training to fix the problems described in the report.

"The agreement between the Postal Service and the union representing sales associates dictates that the Postal Service must select most sales associates through a seniority-based bidding process," the audit said. "Because the Postal Service is bound by this agreement, management has limited flexibility in staffing sales associate positions. However, it can build the desired customer service skills and behavior through continual, formal training."

The USPS dismissed the audit as "irrelevant" and showed no concern about losing customers.

"The 20.5 [percent] used by the OIG, i.e. their percentage of customers not treated ‘with courtesy,’ likely grossly overstates the number of customers who would actually choose a different provider," the agency wrote in response to the audit.