A proposed class of plaintiffs includes borrowers who were 30 days or more delinquent on loan payments when Mr. Cooper acquired their servicing rights. The lawsuit, which sues the firm by its former name, Nationstar, references other recent legal and regulatory actions involving the servicer over other fees it charged consumers.

"While these sums may appear small in the individual situation, in the aggregate they amount to a substantial profit center for Nationstar, paid for by consumers who do not assume they are dealing with a scoundrel," wrote attorneys on behalf of three borrowers who filed the complaint.

Mr. Cooper last week elevated the suit to a Washington federal court after plaintiffs filed it in a state courthouse in February. Borrowers say the fees, which are disclosed on Mr. Cooper's website, violate the Fair Debt Collection Practices Act.

The total amount of fees charged, and overall damages sought, were unspecified. Attorneys for both parties didn't respond to requests for comment, while a spokesperson for Mr. Cooper cited a company policy in declining to comment on pending litigation. 

The servicer is currently facing several class action lawsuits over a data breach it suffered last fall. It also received preliminary approval from a federal judge last November for a $3.6 million settlement in another class action complaint over alleged "exorbitant pay-to-pay fees." 

Regarding the latest fees in question, borrowers allege Mr. Cooper processes payoff fee statements in seconds, and the cost to the company to produce such a statement is "a matter of pennies." The servicer's website says "a preparation fee of up to $25 may apply" to each quote it generates. 

"Other honest mortgage servicers are also harmed by Nationstar seeking and realizing a competitive advantage of imposing and collecting junk fees not expressly authorized by contract or law," the lawsuit said. 

In addition to allegedly violating borrowers' mortgage contracts, Mr. Cooper has also not sought authorization for the fees from the Department of Housing and Urban Development's National Servicing Center.

Two of the lead plaintiffs were in default at the time Mr. Cooper acquired their servicing rights and paid payoff fees. Another Maryland homeowner said the company assessed a fee because the prior servicer, Community Loan Servicing, incorrectly identified the borrower as in default. 

The complaint references the Consumer Financial Protection Bureau's recent focus on junk fees in mortgage lending, although its March blog asked consumers to submit payments related to the closing process. 

Mr. Cooper has a May 9 deadline to respond to the Washington lawsuit.