Charge for postage of multiple items

Hi - can someone tell me what happens in the case of a buyer buying multiple items with this new postage charge?

Say they were each $8.50 postage but would fit in a large satchel for  $13.50, do you get charged the 9.9% on the $13.50 or on each individual post ?    I would of course be even more horrified than I am alrady if it was the latter one !!!    Thanks