Oh, what a shame...
You can use the ibox.tim.it APN also with the TIMxSmartphone Maxi, that's not a problem. You have been charged 5€ for that and you cannot get them back. So use it and you can decide to deactivate it, before 7 days, if you prefer to change to another option.
The problem is that the shop stole you some money, I suppose... you should have payed 10€ for the sim-card plus 5€ for a 5€ refill and have the TIMxSmartphone Maxi plus 5€ of credit. So totally 15€, not 25€. It seems they stole you 10€... welcome to Italy!