There's not really enough information here to be able to give proper advice on (for me anyway) but ill try...
If your using a portal, there obviously exists a relationship.
So, lets say your table was named lineitems (where you entered the towbar )
If you had a field in here for a part number, this could be related to the products database
LineItems -> ProductsDatabase
PartNo = PartNo
By entering (or choosing from a value list) the part number of the wiring kit into the line items table
you could then have it lookup the remainder of details, such as description, price etc.
So for instance, the price field could be a lookup field. When defining the field you could choose the options for
Starting with table 'LineItems'
Lookup from related table 'ProductsDatabase'
Then select price from the related fields.
I'm not sure if this is what your looking for though, so excuse me if I have read it wrong.
got it working ok now not sure what was wrong but ok now
thanks for all your help