how can i make bulb to collect a fixed amount from my account, it doesnt matter if i do not use up to, just collect a fixed amount each month

Ask to pay by direct debit. They use the dd to collect a regular monthly fixed amount. I think it is based initially on 1/12 of your expected annual usage but you can alter the amount if you wish in Account Settings.

But are you sure that bulb aren’t already collecting a fixed amount, either by dd or regular debit card transaction? I didn’t realise there were any other payment options.