If you have miscalculated the duration of the assignment, it is important that you communicate this to the customer as soon as possible and before you continue further work.
Miscalculations can often be the result of several things, but mainly you as the tasker are responsible for extracting enough details before starting work so that the duration is correct.
If there is a disagreement, you may want to read through the terms of use to buy help and offer help to find out where it has failed in relation to the terms.
A solution can, for example, be to meet in the middle, order another tasker at a lower price or that the assignment is completed by the same tasker for an updated duration. This depends on why the estimate was incorrect.