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ADvendio offers many functions which are called by selecting records and then clicking a button. Find a list of all buttons here: 4.2.4 Add Campaign Items / ProductsCampaign Item Buttons and Functions. In some cases you like to overwrite those and add add functionality to those; i.e. add a validations before submitting line items to the adserver.
You can write your custom APEX code and then call the buttons function now by passing all the selected line item record ids now. Ask your ADvendio consultant for more information.