Postage cost s simple postage

[hely69]

Hi there,

I’m very confused with the costing of simple delivery. I’m a seller for example I’ve put £2.70 for a large letter for the buyer, the buyer has paid that amount, but says they have chosen small parcel which is £3.38 so I’m paying out of my own money each time a difference of  68p. For example. This doesn’t need to be a small parcel either.  
why is this happening please help ? 

so in other words I’m seen to be getting out of pocket !

 Thanks in advance,

 kind regards,

H