Abstract: In a document insertion machine, third-party advertising documents are fed from downstream insert stations (36, 37, 38, 39). The third party insert stations include an optional insert station (39) from which documents are to be fed only if the additional weight occasioned by the feeding does not cause an increase in the postage for a customer's stuffed envelope. The other third party insert stations (36, 37, 38) are chargeback stations which feed documents regardless of the impact of feeding upon the weight of a customer's stuffed envelope. The insertion machine apportions, among the chargeback stations (36, 37, 38), any increase in postage cost which occurs when the feeding of documents from the chargeback stations causes a customer's stuffed envelope to be classified in a more expensive postage category.