Hello @Cesare Paro,
As of the moment, there isn't an integrated way to assign a net price to customers. Prices are based on the products or services in each line item as you create the sales transaction.
That said, let's consider manually changing the prices of your products and services the same day you create the sales transaction.
Additionally, I've also included this reference for a compilation of articles you can use while working with us: Manage Customers and Income for QuickBooks Online.
If there's anything else that I can help you with, please let me know in the comments below. I'll be here to lend a hand.
Really sad! doesn't it have such a basic function? I sell wine, for each customer a net price is assigned based on the product and quantity and I would like it to be all automatic. Entering the price manually each time is a waste of time and risk of errors. Let me know if your technicians can upgrade for me.