Textbox attribute, user types in 458, takes the price of the 400-600 category

[jimbeeer]

I have a client who sells window blinds.

 

He wants the client to be able to type an exact width he wants the blind. This then need to be checked against the product attribute of the product and take the range the number falls in and charge the extra amount accordingly.

 

E.g.

 

300-400mm (+£2.00)

401-600mm (+£4.50)

601-900mm (+£7.00)

 

The user types in 420 so the cart adds an extra £4.50 as it falls in the 401-600mm category, but it still passes the 420 through the cart so the owner knows exactly what width to make it.

 

Also, each blind has a 'standard' version and a 'cut down' version. He only wants this to happen on the cut downs.

 

My client is adamant he wants it done like this and it's giving me a major headache so any help at all would be greatly appreciated.