Is it against Ebay rules to state you combine postage and then after the item has sold not do so?

 
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You may be misunderstanding what combined post is.

A seller can combine two or more items into one parcel and then post, but they don't have to discount the post cost for two or more items. Maybe the combination of more items puts the cost into a higher bracket and they can't send for the same price as the single item.