In contrast to concrete fixed price offers, cost estimates are generally non-binding, even if they are part of a contractual agreement, invoiced and signed by the customer. You as a business owner, do not assume any liability for the accuracy of the information in the cost estimate. This means that if the actual costs are higher than expected due to unforeseeable events (e.g. delayed deliveries or price increases), the estimated costs may be exceeded to a certain extent.
However, not every customer is aware of this circumstance, which is why you should explicitly point this out to them – include a sentence such as “this cost estimate is non-binding” should be enough. This not only creates mutual trust, but also covers you in the event of a dispute. Alternatively, you can limit yourself to giving approximate figures in the cost overview – amounts exact to the penny are rather unusual for cost estimates anyway.