I have just upgraded to a new sim only plan and the App tells mr that I will be charged £7.59 on my next DD, which I understand reflects a £1 extra on my monthly payment plus a small adjustment to reflect a short period of upgrade from my previous plan. This I can understand. 
HOWEVER I have also had an email from them saying that they are also going to charge me £3  for my New Plan on the same date as my earlier DD’s, and I CAN’T UNDERSTAND WHAT THIS PAYMENT IS FOR. Can anybody out there explain this??