Dallas from the social care team here to help.
If you are wanting different shipping prices for different weights you would need to set up weight based shipping rates. That can be done under Setting > Shipping in your Store Admin. From there you can set up the parameters for shipping price #1 to be associated to variant #1 (250g) and shipping price #2 to be associated with variant #2 (1kg).
Just be sure that you include the weight of the box as well.
This way, when your customer adds the product to their cart and they start to go through the checkout they will see the correct price that you are wanting to share.
If, however, you are wanting to create shipping rates per-product there are two other ways that I would suggest looking into. The first would be to get an app like Better Shipping or Parcelify. From there you can create product bases shipping rates.
With that in mind, I would suggest my second option instead because it is completely free. Instead of having a shipping option you would just incorporate the shipping into the price of the product variant.
If product #1 (250g) cost $20 and the shipping was $5, you would charge your customers $25. This way you are always receiving the money for the shipping, you can set it up per product variant, and you don't have to pay for an app.
I hope these options help you out, but if you have any more questions please don't hesitate to let me know.
I just want to clarify to make sure that I'm following what you're referring to exactly. Are you just wanting to change the weight per variant or do you want to set up shipping rates per item?
An example for the second option would be if Item A costs $3.00 in shipping and Item B costs $2.50 in shipping. When both are added to the cart the shipping rate would be $5.50. Does that make sense? If that's what you're wanting you'll need to look into an app that can do just that. I have a few that I think could work for you. Advanced Shipping Options, Parcelify, or Better Shipping are all great options!
If you're in fact just looking for information on how to add the weight of each variant I can certainly help with that also. If you go to your Product page and click on Edit then you can make any adjustments in there. It'll look like this:
Does that help to answer your question? If it doesn't please let me know and I'll try and assist you further.
Hi ShipRelay here! Check out our app on the shopify marketplace. If you use our shopify fulfillment services our software's "carrier calculated rates" DO account for changing packaging sizes/variants automatically and works marvels for bundling or international shipping. Check out this video of our app in action found in the shopify marketplace here.