Hi, I have a silver product at £70 for 12 months, gold product at £80 and wish to introduce a Platinum at £120 What is the best way to enable a Silver or Gold user to upgrade to Platinum for the difference? I.e. Silver to Plat: £50 and Gold to Plat: £40 I see there is a surcharge option but that seems to add an odd number (cant work out how this calculates the upgrade amount!). Any help/advice?