Variable costs rise and fall with the number of units you sell. Materials, packaging, shipping, payment fees, and per-order labour belong here. Sell twice as much and these costs roughly double. Sell nothing and they drop to zero. Variable costs decide how much of each sale you actually keep.
List variable costs per unit, not per month. That is the form you need for pricing and for your contribution margin, and a generated business plan shows them per unit beside your selling price.
An example for a candle maker. Wax €2.10, wick and fragrance €1.40, jar €2.30, label €0.20, box €0.90, shipping €4.20, payment fee €0.60. The variable cost per candle is €11.70. You sell at €29, so €17.30 stays in the business to cover rent, your salary, and profit.
Founders get three things wrong here. First, they forget the small items. Payment fees, return shipping, and packing tape look tiny, but €1.50 per unit across 3,000 units is €4,500 a year.
Second, they assume the cost per unit never changes. It usually falls with volume: buy 5,000 jars instead of 500 and the price per jar may drop from €2.30 to €1.70. State which volume your figures assume.
Third, they treat a part-time worker as variable. If someone receives a fixed monthly wage regardless of orders, that is a fixed cost.
The border between fixed and variable is not always sharp. Electricity in a workshop has a base charge plus a machine-dependent part. Split it when the amount is large, and pick one bucket when it is small. Being consistent matters more than being perfect.
