Ash here from the Shopify team. Welcome to the Shopify Community!
I understand you'd like to add the Carrier Calculated Shipping feature to your account, but don't wish to change the plan you're on. That is absolutely possible, and I'm happy to help out with this! I'll need to authenticate you on the account in question in order to make any changes, so I've sent an email to the email address associated with your Shopify Community account for verification. Please reply back to that email and I can further assist!
To recap the options when it comes to enabling the Carrier Calculated Shipping feature to your account, here are the ways we offer this:
I've re-iterated these options in my email to you as well, and you can let me know in your reply which option you'd like to proceed with.