Why does my basic postage rate of £2.94 just to £13.11 when customers in the UK see it?

[thevintagedencd]

I have had a number of occasions where I put my standard basic rate postage of £2.94 then for some reason this shows to buyers as £13.11 for a tiny item. Some have gone as far as paying this. I am in the UK and the customer is also in the UK. Any ideas? Thanks