Hi Fabio,

Catalog Foundation Explained: Product Definitions explains how to set up definitions and data types. The Catalog Structure overview video explains some of the same concepts, but in video form obviously :)

You can absolutely influence the product price on the order line and you don't have to set up a definition to do so, although it would make sense if you want to lock in the values the customer can select.

It's not possible to override the price on an order line from XSLT so you'll have to do a bit of .NET to get there. Here's an example of how to do it:

// Get basket
var basket = TransactionLibrary.GetBasket().PurchaseOrder; 
var orderLine = basket.OrderLines.SingleOrDefault(x => x.Sku == "MySKu" && x.VariantSku == "MyVariantSku");

// Set the price of the order line
orderLine.Price = 100m;

basket.Save();

// Execute basket pipeline to tally the new totals
TransactionLibrary.ExecuteBasketPipeline();

Hope this helps.