For months, the reasoning behind Costco (COST 0.22%) and American Express' (AXP -0.53%) pending divorce remained shrouded in mystery. But with the passage of time, details have emerged that shed light not only on why the two parties decided to go their separate ways, but also on the likely impact the fissure will have on American Express' top and bottom lines. What follows are 10 important things we've learned about this split.

A trial run in Canada
1. Starting in January of this year, American Express cards were no longer accepted in Costco's Canadian outlets. In the first quarter since this took effect, the amount of money spent on American Express cards in Canada fell by 17% -- referring specifically to the company's "proprietary consumer and small business billed business."

2. This has had such a material impact because 60% of the money spent on Canadian Costco co-branded cards came from Costco purchases, as opposed to purchases made outside the store.

Expectations for the U.S.
3. In the United States, by contrast, the opposite is true. Namely, holders of the Costco co-branded card billed 70% of their total charges at other places and only 30% at the membership warehouse.

4. This isn't to say American Express won't be affected significantly when the deal in the U.S. expires at the end of next March, as 20% of its global loan portfolio stems from the co-branded card.

5. Along these same lines, the Costco co-branded cards in the U.S. accounted for about 8% of all charges processed on American Express cards worldwide and roughly 10% of physical American Express cards currently in force.

Why Costco walked away
6. The relationship ended because American Express wasn't amenable to new terms Costco proposed. "The numbers didn't add up," American Express CEO Kenneth Chenault said at last month's investor day. "We couldn't accept their financial terms or their contract terms, some of which would have meant taking on more risk than we were comfortable with."

7. The issue boiled down to interchange fees. Under its existing deal, Costco paid American Express 0.6% of each transaction. This, along with the associated loan portfolio, funded the co-branded card's cash-back incentives, which ranged from 1% to 3% of purchases.

8. By contrast, American Express' replacement, Visa, agreed to charge an interchange fee of only 0.4%, or a third lower.

9. As a further point of reference, according to a 2014 study by the Federal Reserve Bank of Kansas City, Visa's typical interchange fee is in the neighborhood of 2%, or five times the rate it has purportedly agreed to charge Costco.

10. The net result, unsurprisingly, is that American Express' shares have taken a drubbing over the past year. At present, they're down by 19.2% from their 2014 high.

A tale of two companies
It's hard to say, of course, what the end game is for American Express. Its executives have steadfastly said they will make up for the pending loss through alternative growth initiatives, revolving principally around marketing. Additionally, given its shrewd and successful past, there's reason to believe American Express when it claims this was the right decision for the credit card company over the long term.

For Costco, on the other hand, it's hard not to be optimistic. Yes, the change is small. And yes, Costco is already arguably the most promising retailer in the United States today. But for a bulk retail chain that thrives on low prices, cost savings like this will only add to its ability to attract even more budget-conscious customers.