Sales Order Quantity and Pricing Tollarance

Hai Experts,

        We are chemical trading company.for sales process we are facing tollerance issue,described followes

if a person order 'X' Chemical-18000 lit qty and unit price is 50Rs

1.when a person fill the 'X' Chemical from Jetty(Tank Bay) to Tanker Lorry manually not Possible to fill exactly ie 18000qty

it may be 17500 or 18500litre.

2.for this variation price should't varry ie  price is changing when i change the quantity.

3.So we need Tollerance for sales order

                  Pls help me to sort out this issue......

Thanks in Advance

Saravanan V