Paid final bill Eon

Hello Paid final bill to Eon and now a bulb customer . Bulb took first payment,I have got an email from Eon saying account closed and outstanding bill£150.00 to be paid.
I am confused to why any advice please

You pay Bulb for the month in advance.

Your final bill for Eon will be based on your final meter readings.

Yes, @phproxy is exactly right. We take payments in advances while it sounds like you were paying EON in arrears.