Nick here from Shopify.
Really good question. After reading your question, I feel the best workaround for the two scenarios you presented might not be another shipping label for the order but by creating a draft order. Once you have created a new draft order for the instances where the scenario's above occur, you can create and apply the new shipping labels to those.
There are a couple of apps in the Shopify app store which might be useful when it comes to the actual shipping label aspect of your orders which you can see below:
You can also find the Shopify help guide on Shipping Labels here.
Another way you can do this also is by breaking the order up by product. For example, if someone purchases multiple products from your store you can edit their quantity and break up the shipping labels as well as offer individual tracking numbers should you wish. Once in an order with multiple products, you will see something along the lines of the image below:When you click on create shipping label it will give you an option to select the quantities for each product. You will click zero for the products you want to break away from the first label/ tracking number and continue the process to make as many different aspects of the order as you need. Once you fulfill the item it will give you an action item where you can print a unique packing slip for each part of the order which you can see below: I know there is a lot of information and different methods around this. But I do believe with the scenario's you asked about originally a draft order is the best way forward. But I also wanted to show you how to create multiple shipping orders for one order also.
Hope it all helps and don't hesitate to ask any other questions should you have any.
All the best, Nick
These work arounds may work but they do create many extra steps, each of which open up opportunities for human error like entering a customers information wrong in a draft order (happens all the time). Shopify should absolutely listen to this problem and add a simple "create additional label" button which allows seller to make new labels with new shipping details like box size and weight while automatically filling in the buyers other info. This isn't a question of there being a work around or not, this is an incredibly useful tool that would be easy enough for Shopify to add and it should. I used to use a similar feature on Shippo all the time until they eliminated it. Now I have to manually enter peoples information and screw around with layers of complication for absolutely no reason other than I have to work around a poorly designed system. It has the ability, it just doesn't care. This is not a rare problem, it is a stupidly easy thing for Shopify to make that would solve a very common and unnecessarily complicated problem.
I need to use this feature right now, twice, for two different orders because the shipping warehouse accidentally fulfilled all the items when one of those items (which must ship as 2 separate packages because of size and weight) when they did not have all the bits to ship at that warehouse. These small features are what make the sellers on your platform more efficient, which makes their businesses run better, which brings in more sales, which directly benefits Shopify. Its not just a kindness, it literally helps your bottom line and you have people on staff already that could know this out in a couple hours. I love Shopify, I hate inefficiency and unnecessary complication.
TLDR: Just make it happen Shopify, we would all benefit. Build the best product you can especially when its so simple to make better.