Select weight of the product during checkout

[Gaddy]

I have a problem. Well i have 3 Facewash products one is 100gm , second is 50gm and other is 150gm.

 

Now i during checkout in the cart we can increase or decrease the product quantity by inputting the number. For example we can change from 1 product to 2. But what if i want to change the weight? Like if someone buys a 150gm product , I want an option ..... so that he can change it to 100gm or 50gm on the check out page itself? Thanking you in advance.