Concentrates and dilution
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The format that fixes the shipping problem and introduces a usability one.
The commercial argument
A ready-to-use cleaner is mostly water, and shipping water is expensive, heavy and worthless.
A concentrate ships a fraction of the weight for the same number of cleans, uses a fraction of the packaging, and occupies a fraction of the shelf.
It is also the strongest environmental claim available to a small maker, and one that is factual rather than a Green Guides problem.
What changes in a concentrate
Everything is closer to its solubility limit, so the formula is less stable and more likely to separate or to drop out in the cold.
The preservative has to work at concentration and the diluted product still has to be protected, which is a different problem from a ready-to-use formula.
Hazard classification can change: a concentrated version of a mild cleaner can be a hazardous substance requiring cautionary labeling, and can cross a child-resistant packaging threshold that the dilute version does not.
That last point catches people. The 2 percent hydroxide threshold applies to what is in the bottle, so a concentrate can need a CR closure while the ready-to-use version does not.
Telling the customer the ratio
Make it unmissable and make it easy. A ratio in milliliters per liter is precise and nobody measures it; "one capful to a bottle of water" is imprecise and everybody does it.
Give both, and design the closure so the cap is the measure.
State what to dilute into as well: their water is the other half of the product, and hardness will affect the result.
Where it goes wrong
Under-dilution, which wastes product and can damage a surface.
Over-dilution, which produces a weak result the customer blames on the product.
A refill bottle that gets topped up without being emptied, which is how a dilute product becomes contaminated.
And the customer who tries the concentrate neat to see if it works better, which is why the hazard labeling on a concentrate has to assume undiluted contact.