Break-even CAC is the most a store can spend on advertising to win one order before that order loses money: what the order leaves after its goods, shipping, packing and payment fees, its CM2. Below it an order pays for its own acquisition; above it the order is bought at a loss, and only a repeat purchase can earn the difference back.
It turns a margin into a budget. An order of €80 net revenue that leaves €36 after its goods, its parcel and its payment fee can carry up to €36 of advertising and still break even; spend €45 to win it and the store is €9 down on the sale before rent or salaries see a cent. It is the same CM2 that sets break-even MER and break-even ROAS, stated in money per order instead of as a ratio.
Measured per product it gets sharper, because CM2 differs widely across a catalogue. A cheap accessory in a heavy parcel may leave a few euros while a bundle leaves several times that, and a budget that treats them alike overpays for one and starves the other. Set it beside what acquisition actually costs, total marketing spend over new customers, and the gap says whether first orders pay for themselves or depend on customers coming back.
Repeat purchases change the reading rather than the figure. A store whose customers return can spend above break-even CAC on purpose and earn it back on the second order; whether they do, and after how many months, is what the LTV tab measures and what the CAC payback calculator estimates from your own averages. Spending above it without knowing the repeat rate is lending to customers with no repayment date.
In nouz, Insights, Products gives every product its own Breakeven CAC: its CM2 divided by the orders that contained it, with the costs below CM1 split to products by what drives them. Insights, Customers sets the average first order's CM2 beside blended CAC, which is the same test for the store as a whole.