Application Express :: 4.2 - Validate Field In Tabular Forms?

1.- i have a product_item field (wich is a lov and user choose one value), so i need to call a function (sending it the selected value product_item) that will return me some data about the product i selected, for example :

if the function return 'A' then
disable the next item on tabular form, lets say quantity
else
enable the next item on tabular form, lets say quantity
end if

2.- i need to determine the result of two items operations:
total := final_quantity - start_quantity
the user will introduce start and final quantity and i will determine the total.