You mean EVERY time you topup/buy membership?
You seem to be using an iPhone?
If so then these have an unfortunate habit of sending UK SMS messages behind the scenes to activate certain services and each of those costs 25c.
This is down to the stupid way that Apple do things unfortunately.
Conect 48 support and ask them where the 25c is going and it should clarify matters.
See the link in my signature below.
Hope this helps.