public decimal GetServiceCost(Webshop webshop) => Income * webshop.OnlineServiceCostPercentage / 100;
public decimal GetCost(Webshop webshop) => GetProductCost(webshop) + GetServiceCost(webshop);
public decimal GetProductCost(Webshop webshop) => Income * webshop.ProductCostPercentage / 100;