For many products the default unit of sale is a single item, however for others the default unit of sale is a multiple.
Where the unit of sale is a multiple, for example a quantity of 100, this is generally the default quantity added to the cart. In general you can buy a smaller quantity than this default, however the price per unit of sale will be re-calculated if the quantity is changed. This re-calculation is occurs when you change the quantity in the shopping cart and click Update.
We use a price calculation designed to deliver to our customers fair and sustainable prices for the chosen quantity.
Typically for a small quantity of an inexpensive product you will be offered a price closely related to the total cost of sale (stocking activities, picking and billing), whereas for a larger quantity you will be offered a price more related to the cost to us of the product itself. In the first case other costs are the most significant, and in the second other costs are least significant as a proportion of the sale.