If you are using the standard ADvendio settings the AdServer API will always submit the value out of your amount n3 field (ADvendio__Amount_net3__c) out of your line item. You can switch this by following these steps:

Navigate to your Adserver login at Overview - AdServer Logins - and choose your Adserver Record: Make sure the field AdServer Price Field is in the layout. There you find three options:

If you choose

1. Adserver Price the system will submit whatever you have written into the field "Adserver Price"  

   Adserver Price	ADvendio__AdserverPrice__c at campaign item level

   within your campaign item. You can build a process, workflow or otherwise automate it with which value you like to fill it. If NULL (undefined) we will submit 0.0.

2. Amount net/net  will submit the value of 

   Amount net/net	ADvendio__Amount_net_net__c divided by ADvendio_Quantity__c - at campaign item level

   to the AdServer.

3. Amount net3  will be the same as the standard and submit the field value of

   Amount net3	ADvendio__Amount_net3__c divided by ADvendio_Quantity__c -at campaign item level