I tried it on a car once. Started low and worked up. The knock back was almost instant so it has to be automated as if a lower amount that the seller will take has been programed in. I did the same with an engine cover. Started at $110, then $120 and then $130 which was accepted. So find out postage from seller and as others have said, if the postage is for eg $20, offer $20 less and post is free in affect .