Made a mistake with postage, what can I do?

swingaloo

I sold an item and for the first time ever I undercharged on postage, my fault and Im fine with paying the difference myself. 

Its a quite fragile item and I had the weight and box but on packing it is evident that I need far more bubble wrap than I first thought. Of course this makes the parcel a lot bigger. 

It is simple delivery so I have the pre paid label but it does not cover the parcel size. Can I buy additional postage at the PO to add to it or is my Ebay protection not going to be covered now if I do that? 

One way or another I need to increase the postage price myself as its my fault but the item is expensive and fragile so I cant risk less packaging. 
Is there a way to do this please?

swingaloo

Please ignore the above. 

Just to clarify should anyone else have the same problem- I mistakenly charged for a small package rather than a medium. Not the buyers fault so I wanted to put excess postage on but was concerned I may not be covered if I did not use the original label.
I went on Ebay chat- asked for an agent and only had a 1 minute wait. 

Problem sorted, just told to use the label I have and that because of the agreement they have the Post office must accept it. I will still be covered if any issues and my buyer cannot be charged for underpaid postage. Got a transcript of the conversation and verification within minutes.

On this occasion Ebay were very helpful.