i queired (sp) this with thomas cook as we only got 15kg off them but fly with Iberworld. I looked on the Iberworld website and it says we're allowed 20kg's and thats what they issue you with, but its the travel agent that takes 5kg's and sells it back to you if you want more.
I wrote to ABTA about it because i didnt think it was right that i have to buy back what was issued to me in the 1st place! anyway long story short thomas cook have now allowed us the 20kg's that Iberworld give us.
Ask your travel agent, i did, why should i pay for something that was mine anyway!