This article was originally published on March 8, 2015, and updated on July 30, 2015.

It's hard to get over a big divorce. Just ask American Express (NYSE:AXP), which is the exclusive credit card brand accepted at Costco Wholesale (NASDAQ:COST) stores in addition to being the popular retailer's sole co-branded partner for its TrueEarnings credit card.

But not for much longer. This past February, the two companies failed to agree on terms to renew their arrangement -- which was a long and generally happy marriage that lasted for 16 years.

Now that we've had a few months to sift through the ruins of this relationship, which firm or firms emerged from it in the best shape -- the retailer, its replacement partners, or the ex-spouse?

Quickly back to the altar
Costco didn't waste much time finding new mates. Less than a month after the split with AmEx was announced, the retailer drafted Visa (NYSE:V) to be its exclusive credit card brand, and Citigroup (NYSE:C) as the co-brander of its proprietary card. The new deals will come into force at the beginning of next April, just after the AmEx arrangement expires.

Since the divorce was made public, AmEx's stock price is down by over 5%, below the flat-lining S&P 500 index, and significantly under the 12%-plus of both Visa and MasterCard. Costco exclusivity is considered a prize, and losing or winning it is taken seriously by the market.

Ultimately, though, there's one big winner in this contest, and it's not a credit card company.

It's since come to light that Costco will pay almost nothing in fees to Visa and Citigroup as per the new arrangement

A report in Bloomberg, citing "people familiar with the arrangement," has it that the retailer's costs of accepting Visa and Citigroup plastic will be very close to zero. This compares quite favorably to the current AmEx rate, estimated at around 0.6%.

It also, perhaps, helps explain why the "don't leave home without it" firm was willing to leave the Costco arrangement. It's not very appealing for a business to offer a service nearly for free.

Of course, near-zero isn't quite zero. Just on pure purchase volume, Visa will draw a decent revenue stream from the deal. It's just not the slam dunk it might have appeared to be when the new arrangements were announced.

The situation is a little different for Citigroup. Since it'll be financing the Costco cards as the issuer, it'll reap interest payments on card holders who carry a balance. And when those plastic rectangles are used outside of Costco, the bank will take in fees from those sales more in line with its typical rates.

Credit to Costco
Meanwhile, hooking up with its new partners gives Costco a direct pipeline to a much wider pool of credit card holders. As of the end of 2014, there were 304 million or so Visa credit cards in circulation in the U.S., far higher than the 55 million AmExes. There's a clear and obvious benefit to accepting that many more cards.

Additionally, linking with Citigroup -- a very busy and assertive card issuer -- could also grow the numbers of the retailer's proprietary card portfolio.

Of course, at least some of this depends on what happens to the current TrueEarnings holders. After all, they're technically AmEx's customers, and it remains to be seen whether the card giant will sell all/some/none of this card book, or retain it in the hopes of moving existing holders to new products.

All in all, Costco/AmEx is a major divorce that will continue to have repercussions for all parties concerned. We'll keep an eye on developments stemming from this situation, and update them going forward.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.