Insufficient Postage

[terminalboredom]

I have sent two items out to buyers using eBay prepaid labels and both of them have been delayed by Royal Mail due to insufficient postage.  How can this be?  What can I do about this as the buyers are understandably annoyed.

[terminalboredom]

How do I get in touch with ebay about this.  ebay supplied the labels but the postage turned out to be not enough.  My sellers need to know when they will get their items - the RM tracking only shows that the items were collected but no further tracking shown.