MOUNTAIN VIEW, Calif. — Verisign Inc., whose products help authenticate people and companies on the Internet, has reached an agreement to make its Payflow service available to American Express Co.’s online merchants.

The arrangement with the card company will let the merchants authorize and settle transactions directly with Amex in real time without having to pay a per-transaction fee.

American Express processes its own payments, but merchants typically use third-party processors to route the transactions to the card company. This costs up to $1 per transaction, said Amer Deeba, Verisign’s director of marketing for payment services.

Payflow charges a flat fee for merchants to connect to its service. Smaller merchants pay a $179 setup fee, plus $20 a month. Larger merchants pay $249 for the setup, and $60 a month for up to 5,000 transactions. A merchant can get unlimited transactions for a monthly fee of $995, Mr. Deeba said.

Payflow, he said, is the only flat-fee payment service available to online merchants.

Mr. Deeba said Payflow’s real-time capability allows for quicker authorizations than processors that submit payments to American Express in batches every few minutes.

As part of the deal, he said, American Express will promote Payflow to its small and midsize merchants in the United States and the Caribbean.

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