Marco 

Please try and understand that Royal Mail and Post Office Counters are two different companies. ebay may have a deal with RM but I can assure you it does not have a deal with the post office!

I am not even sure if ebay realise that the post office, and royal mail are two separate companies as I know no sub postmaster would accept a wrongly pre-paid label on a parcel, Each post office would be fined by RM for this breach and part of their monthly targets on correct segregation. Every post office employee would say either get a new label or buy the correct postage. You cannot pay the difference over the counter. We wait with baited breath........