Thank you for posting to our forums!
We have a solution that should work for the situation you described. You can create a Shipping Rule to apply a Saved Carrier Selection which includes packaging.
First, set up your Saved Selection(s): How to: Save my Carrier Selections for future shipments
Then, create a ShippingRule that uses your Saved Carrier Selection: How to: Create a Shipping Rule
You can see Saved Carrier Selection is listed in the available THEN actions:
If you need further information about Shipping Rules, I recommend taking a look at this article from our Knowledge Base: FAQs: Shipping Rules / Mapping
Since you are currently on a paid plan, you can use the links at the top of the app to contact us with any further questions. Thanks!