[mothered]

If i buy 14 inch laptop is it important that donor laptop should be same size (14 inch) and the same weight ( max 120g difference)?

As long as the wrong Item fits In the same package as the original Item, you'll be fine.

In terms of the weight, allow for a 100-120 gram variance.

If i buy two expensive phones can i change them with two cheap ones? Or wrong item method only works with one item?

Stick to the one Item.

It's unlikely a picking & packing error was made with 2 Items on the one order.