I have just sold multiple items on a multiple item listing, but due to Simple Delivery, postage has only been paid for a single item meaning the parcel has gone out well overweight. Item postage label is prepaid due to simple delivery, but is not covering the multiple items purchased weight, and I have a feeling im going to be stung for this. How can this be avoided on future listings, as I can't see a means of charging additional postage? Is this a flaw in the system?

Additional postage for multiple item listing purchases | Member To Member Support

Additional postage for multiple item listing purchases | Member To Member Support | eBay UK