Modeling question re: multifaceted price calculation

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

Fellow UBL-Dev'ers,

I'm trying to determine which UBL constructs to use to model the 
invoice line items in the following information:

Invoice period:    April 1, 2008 to April 30, 2008

Item:              1
Description:       Thing-one
Quantity:          7
Base cost:         $1000.
Rental rate each:  2.5% per month
Shipped to site:   April 15, 2008
Removed from site: April 20, 2008
Days on site:      5
Rental fee:        $29.17      (=1000*.025/30*5*7)

Item:              2
Description:       Thing-two
Quantity:          3
Base cost each:    $2000.
Rental rate:       $30 per month
Shipped to site:   April 10, 2008
Removed from site: N/A (still on site)
Days on site:      20
Rental fee:        $60         (=30/30*20*3)

Item:              3
Description:       Thing-three
Quantity:          1
Base cost each:    $5000.
Rental rate:       1% per month
Shipped to site:   N/A (was on site)
Removed from site: N/A (still on site)
Days on site:      30
Rental fee:        $50         (=5000*.01/30*30*1)

I'm tempted to put a lot of the line item detail into an extension, 
but I would rather find UBL constructs that express the many facets 
of the price calculation.  All of the items need to be found in the 
instance since the printed invoice needs to reflect all the fields.

Thanks for any guidance on which UBL business objects to use!

. . . . . . . . Ken

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