In order for shipping providers to accurately calculate how much something will cost to post, you'll need to supply it with some information about each product. Specifically, the dimensions and weights for each product must be set up. Without these set, you'll likely get inaccurate results, or none altogether!
Fortunately, Postie has an easy way to let you know which products or variants you need to update with dimensions or weights. Navigate to Products under the Postie settings to be shown a list of products or variants that do not have these values set up.