If the products are shipped from different warehouses, it is necessary to pay for two shipments. If the goods are sent from two different warehouses, from different places in a given country, they need to be delivered by 2 couriers.
Most stores include these costs in the product price or slightly increased delivery price (especially since most orders concern single items). Transport costs should not be regarded individually, but statistically, e.g.
If 20 out of 100 orders in a month are from different warehouses, there are 120 shipments. So the average shipping cost equals 120% of the unit price.
This means that a shipment costs on average not 15 PLN but 15 PLN + 20% which gives 18 PLN. This 3 PLN should be treated as a fixed cost of order processing.
The above numbers are only examples and individual costs are determined by numerous factors - it is possible to analyze detailed data only after a few months of activity and cooperation.