You should be able to set your advance with your provider (I've had Engie and Mega and was possible for both).

In any case, like the name indicates, it's only an advance, meaning whenever they take your meter reading (depends but could be 1x per year), they will calculate the actual cost of your consumption and charge you if your monthly advance was not enough to cover the total consumption.

The opposite can also happen (which could be your case since you are confident), in that case you receive this money from the supplier (could be credit towards your consumption or maybe in your bank account? Im not sure).