BNPL Under the Hood
Klarna pays the shop today, you pay Klarna over six weeks, nobody charges you interest. So where's the money coming from?
The walkthrough
Buy Now, Pay Later (BNPL) is the "Pay in 4" button at checkout, typically a quarter upfront and three more instalments every two weeks, with no interest to you. The classic product is *Pay-in-4* from Klarna, Afterpay/Clearpay, Affirm, PayPal Pay in 4 and Zip. It feels free, so the obvious question is who funds the gap, and the answer is the merchant, not you.
The merchant pays. When you check out, the BNPL provider pays the retailer the *full* price almost immediately, minus a fee. That merchant discount rate (MDR) is roughly 2–8% of the basket, much higher than the ~1–2% a card costs. Retailers accept it because BNPL lifts conversion and average order value (the "split it into 4" nudge), so they trade margin for more, bigger sales.
The provider carries the risk and the float. The provider has now paid the shop but is owed by you over six weeks. It funds that gap with its own balance sheet, warehouse credit lines from banks, or by packaging the receivables and selling them on. Its core job is underwriting: a fast, soft-credit-check decision in milliseconds at checkout, approve or decline before the page loads. Defaults and fraud are the provider's loss, not the merchant's.
Where the money is made. On interest-free Pay-in-4 the revenue is the merchant fee plus late fees (often a few pounds/dollars per missed instalment, usually capped). The richer margins come from longer-term financing, 6, 12 or 24-month plans (e.g. Affirm) that *do* charge APR, sometimes 0% promotional but often 10–36%. That interest-bearing book is where providers like Affirm earn most of their money.
The catch for you. Missing a payment can trigger late fees, a referral to debt collection, and, increasingly, a mark on your credit file. From 2024–25, UK and US regulators (the FCA and CFPB) began pulling BNPL toward the same affordability and disclosure rules as credit cards. The product is genuinely interest-free at the front, but it is still credit, and the failure mode is stacking several "free" plans you can't collectively afford.
The rail map
The Pay-in-4 rail
- 1Checkout
You pick "Pay in 4"; the basket and your details go to the BNPL provider.
- 2Underwritinghighest cost / risk
A soft credit + fraud check approves or declines in milliseconds before the page loads.
- 3Provider pays shop
The provider sends the retailer the full price now, minus a 2–8% merchant fee.
- 4You repay
25% upfront, then three instalments every two weeks, interest-free if on time.
- 5Default / latehighest cost / risk
A missed payment triggers late fees, collections, and a possible credit-file mark.
Glossary
Pay-in-4
The standard BNPL plan: 25% at checkout then three equal instalments every two weeks, interest-free.
Merchant discount rate (MDR)
The fee a retailer pays the BNPL provider per sale, roughly 2–8%, well above card costs.
Underwriting
The instant approve/decline decision the provider makes at checkout, usually via a soft credit check.
Warehouse credit line
A bank facility a BNPL firm draws on to fund the cash it has fronted to merchants.
Receivables
The money customers owe the provider; these can be bundled and sold to raise funding.
Late fee
A flat charge (often capped) applied when you miss a scheduled instalment.
Check yourself
1.On interest-free "Pay in 4", who funds the gap between the shop being paid today and you paying over six weeks?
2.Why do retailers accept a 2–8% BNPL fee that is far higher than the ~1–2% a card costs?
3.Where do BNPL providers earn their richest margins?
4.In the Pay-in-4 flow, where does the most risk and friction concentrate?
5.What is the main consumer danger regulators like the FCA and CFPB are worried about?