The usual process involved with reaching a price on work to be performed is twofold.
After initial consultation an estimate is provided to see if the work requested is within the budgetary constraints of the client. If the work is to proceed, the final design details are then worked out and a firm price is given including taxes.
From here a delivery time is estimated and a 50% deposit is required for the work to commence. No further payment is required until the job is complete and the customer is satisfied with the final product.
The only time there is a possibility of increased charges is if there are unforeseen and /or uncontrollable issues involving an installation. These situations are rare and usually only occur with kitchens or other more complex built-ins.
All work is warranted for life and any repairs or replacements will be done at no charge. This does not cover wear and tear caused by the use of the client.