Hi fredwerk1984
For example, I have 1 bag of 25 KG cereals, each time a customer purchase 5 KG, I want the inventory reduce and becomes 20 KG
Sure, with the Product Levels add-on, you can create a BOM product for your Raw Material and assign it to every final product that uses it in the quantity you need to be discounted. And when sold, the inventories will be adjusted.
Also is there a better solution to variable products? Would your plugin be able to calculate £1.83 per 500G and can multiply if customer choose higher amount?
You can assign BOM products to simple products or to variations.
But bear in mind that if you are going to sell in grams, the BOM products should be set up in grams. So instead of creating a bag of 25kg, you should create a bag of 25000gr.
Anyway, the best way to check if it suites you is by trying the add-on. You can install a trial and test it out on your own site for 2 weeks.
Best Regards,
Salva.