The network commission is constantly changing, and depends on the load on the network. Its value at the moment, you can see, for example, on the resource:              

blockchair.com/bitcoin

but for your convenience, we included the network commission into our prices for you to make it convenient for accounting and work with constant numbers, instead of constantly monitoring the network commission at a given moment of time, and make a correction for the value of your goods in BTC for the buyer. Also, you can see the network commission in the API-request of GCP.