When you are ordering multiple items of the same product but with a different design in the same order, the product pricing is very simple as it is simply the unit price multiplied by the number of items.
E.g. in the example below, 1 Classic Matte Paper poster in A4 format is EUR 6.18. This means that if you place an order for 2 Classic Matte Paper posters in A4 format with different designs the product cost will be EUR 6.18 x 2 = EUR 13.62.