Raw materials, packaging, shipping, payment processing fees and hourly labour are all variable costs. They are zero when you sell nothing and grow linearly (or nearly so) with each additional sale. Together with fixed costs, they determine total cost at any volume.
The per-unit selling price minus the per-unit variable cost is the contribution margin — the cash each sale contributes to fixed costs and profit. A negative contribution margin means every sale loses money, no matter how many you make; the price must be raised or the variable cost cut before scale helps.
In practice, variable costs are rarely perfectly proportional. Bulk discounts, learning curves and capacity constraints introduce step-changes. For planning, a weighted average over the expected volume range gives the most useful number.
An online retailer pays EUR 6 for product cost, EUR 2 shipping, and EUR 1.50 payment processing per order. Variable cost per order is EUR 9.50; at a EUR 25 sale, contribution margin is EUR 15.50.