Shopify natively allows the administrator to set up what dimensions will be used in shipping. However, when using ShipStation integration to receive calculated rates, SS does not take the dimensions data and instead just applies a 1x1x1 to provide Shopify the rates. This returns inaccurate rates in some circumstances where dimensional weight is a factor. The dimensional data is is there on Shopify, but through a conversation in support chat with both Shopify and SS I learned that ShipStation is not currently set up to handle that data. It would be great if that connection could be made.
This makes a great deal of difference when talking about Dimension Weight and its impact on 2nd day air / next day air service levels. The band-aid solution is to go into SS settings for the selling channel and set a % modifier to adjust the price that SS sends to Shopify. I personally ran some tests and used data sets to come up with a good % adjustment for my store; this solution was pretty accurate with a slight margin for error.
But I find the lack of this feature disappointing. The idea of integrating ShipStation with Shopify and enabling Calculated Rates is to get accuraterates. Which, they are accurate for lots of circumstances... just not all. And a major loss of accuracy is due to 3 simple data points: L, W, H. Seems if this data connection is already setup to pass information back and forth, couldn't those be part of it as well? The goal is to get accurate rates, not sure why they were ever left off.