Interchange caps exist. Your card bill may not show them
A small merchant who has heard that card fees are capped at a fraction of a per cent, and who is paying several times that, is not being overcharged in any unlawful sense. The cap is real and applies to one component of the cost. The invoice bundles that component with others into a single rate, and the bundling is where the difference between the headline and the bill lives.
Card acceptance costs have three main parts. Interchange is paid by the merchant's acquirer to the cardholder's issuer. Scheme fees are paid to the card network. The acquirer's own margin covers processing, settlement, risk and profit. Only the first is capped.
What is capped
Regulation (EU) 2015/751 sets maximum interchange fees for consumer card-based payment transactions: its third article deals with debit card transactions and its fourth with credit card transactions, imposing ceilings expressed as a percentage of the transaction value. Those caps are the figures that circulate in merchant forums, and they are accurate as far as they go.
What they do not cover is nearly as important. Commercial cards issued to businesses fall outside the consumer caps. Three-party schemes operate under different arrangements. Transactions where the issuer is outside the area are treated differently again. And scheme fees — which have risen in importance as interchange has been constrained — are not capped by this regulation at all.
Why the invoice looks nothing like the cap
Most small merchants are on blended pricing: one rate, sometimes two, applied to everything. It is simple to understand and it hides the composition completely. A blended rate is the acquirer's estimate of its own costs across your expected card mix, plus margin, and if your actual mix tilts towards cards that cost the acquirer more, the acquirer keeps the difference in either direction.
The alternative is interchange-plus pricing, where the invoice shows interchange as passed through, scheme fees separately, and the acquirer's margin as a stated figure. It is less comfortable to read and it is the only form in which a merchant can see whether the capped component is in fact being charged at the cap. Asking for it is a normal commercial request, and the answer is informative whether or not it is granted.
The transparency obligation nobody uses
The regulation does not only cap fees. It also requires acquirers to provide merchants with information about the fees charged, broken down in a way that lets the merchant see what it is paying for, unless the merchant has agreed otherwise in writing. That last clause explains why so few merchants ever see the breakdown: the simplified pricing they signed up for is the agreement otherwise.
Which makes the breakdown a thing to ask for rather than a thing to expect. A merchant who requests the itemised view and receives it is in a position to compare offers on something other than a headline rate — and headline rates are, in this market, the least informative number available.
What the cap achieved
It is worth being accurate about this, because both triumphalism and dismissal are available and neither is right. Interchange on consumer cards fell where the caps applied. Total acceptance cost for small merchants fell by less, because scheme fees and acquirer margin are not capped and both had room to move. The regulation constrained one component of a three-component price, which is what it set out to do and all it did.
For a merchant, the operative conclusion is unromantic: the capped component is probably being passed on correctly, and the parts of the bill worth negotiating are the ones no regulation touches.