That amount of walk I would hit the customer for about $10.00. $5.00 for seniors. Try to work the cost of the walk into the cost of the driveway. For instance, instead of charging $15.00 for just the driveway, charge $25.00 for the whole thing. If the customer balks at the price, tell him you will include the walk for free. The customer feels he/she got a better deal and you may look like a saint to the seniors.