product weight by price

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.6.5
-- PHP version -- : 7.0.3
-- Browser(s) name and version -- : firefox 47

i sell rice in my shop , i should say that in my country there is custom about buying rice which the customer say price and the seller sell him the rice weight by the price .
now i didn't found a way to make a situation that my customer enter the price and then the hikashop give them a weight . how can i do that ?

how can i write custome code