Invoice Line Items based on Value in the Header.

We have a customer who is considering changing the way the price items and invoice customers.

Here is what they want to do:

On the invoice header create a custom field called Linear Distance.  Based on the value in his field, create line items for the fees.

Then they want the pricing to follow a structured scale as such.

0-5 meters, $500 + $55.55 per meter
5.01-10 meters, $1000 + $75.00 per meter
10.01 - 20 meters, $5000 + $155.55 per meter

Therefore, if a customer purchased a 2 meter widget they would be invoiced $611.10

They would prefer one line item for the total but will be satisfied for the flat rate (e.g. $500.00) on one line and cost per meter on another (110.10).  However they do not want to type the value meters in the line item.