The most common way to activate the Calculated Rate feature on your shop if you are on Shopify Basic or Shopify Regular is to switch your Shopify billing to annual.
However, switching to annual billing alone will not automatically activate the feature.
You will still need to contact Shopify support directly, and explicitly make a request for them to add the feature to the shop.
Contact Shopify support via Live Chat or Phone, and tell them:
"I've switched my account to annual billing, can you please add the Calculated Rate feature to my shop for free?"
How do I contact Shopify Support to request the calculated rate feature?