Which products sell together, and what are they worth?

• Three kits make the five strongest pairs: an espresso machine with a milk jug and with descaling tablets, a teapot with a tea infuser, and a pour-over dripper with paper filters and with a gooseneck kettle.
• The money is in a few offers. Three bring 65% of the €3,495 a month: an espresso machine offered with a milk jug, €1,365; an espresso machine offered with espresso beans, €522; a teapot offered with a tea infuser, €386.
• Selling together is not the same as buying next. Paper filters and a dripper share an order 4.2 times as often as chance, yet customers who had bought filters went on to buy a dripper less often than other customers, 11.1% against 15.4%: most already own one. An offer goes to orders without the second product, so only offers that held this test are priced.
• New customers buy the kit. In a new customer’s first order a dripper and paper filters go together at a lift of 5.2, against 4.1 in returning customers’ orders.

The slices. The shop’s 45,607 customers place about 12,100 orders a month, at an average of €39.99. The rules are mined on the nine months from October 2025 to June 2026; the test months, July to September 2026, are held out. A new customer’s first order is the order in the month they first ordered; every other order is a returning customer’s, and the two add up to all orders. A category keeps the rules whose first product is in it.

Two tests on the months held out. The same-order test asks whether the pair still shares an order more often than chance in July to September 2026, by at least two standard deviations. The later test, the example analysis’s, asks whether customers who had bought the first product by June went on to buy the second more often than all customers who ordered in both periods and had not bought it yet. An offer “holds” when it passes the later test, because an offer goes to a customer who does not have the second product.

How the money is worked out. An offer goes to the orders that hold the first product and not the second: 783 orders a month hold a pour-over dripper without a gooseneck kettle. 16.2% of the orders holding a dripper hold a kettle, against 4.2% of all orders: 12.0 points above chance. The shop made no offers, so how many customers take one is an assumption, set with the slider: here 20% of those points, 2.4% of the orders shown it. At €18.00 margin a sale, that is €339 a month. Paper filters, the dripper’s strongest partner, earn €71: lift picks what belongs with a dripper, margin picks what pays beside it.

What this does not show. Which offers cause a sale. Every purchase in these orders was unprompted, so a customer who buys filters with a dripper does so without an offer, and a discount to them gives margin away. We recommend testing the priced offers with a random half of their customers left without them; the example analysis sizes that test. The prices and margins are the year’s; a new range or a price change calls for mining the rules again.

The full analysis, with the next offer for each of the shop’s main products and the test that would measure what an offer adds: which product to offer next, a market basket analysis.