Soldato
It is 100% a thing. I am on it. I pay by direct debit (the direct debit scheme that banks run) and it is variable. It is 100% a thing.
Again I think its terminology, I am using business terminology.
Again I think for a consumer your asking for an in arrears bill with collection via DD.
That is NOT variable DD in its true sense.
Variable DD infers that either party can amend the value, and or cancel it.