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;