Can you please share some more details on this
Welcome to the Shopify Community! Thanks for posting.
It sounds like you are setting up Shopify Shipping live rates is that correct? If so, Shopify Shipping only uses the default box dimensions when calculating a rate. You can add additional boxes to your settings, however you will have to manually select those boxes after the order is placed. Therefore, we recommend using a package size that is slightly larger as your default if you think customers will be placing multiple items.
If you would like to use multiple boxes to calculate the shipping cost at checkout based on packages other than your default package, then you can use a shipping app.
A third option is to reconsider your shipping strategy. For example, instead of calculated rates, you can use flat rates such as $5 shipping. Or, you could even provide free shipping and increase the cost of your products to cover the expense of free shipping. We have a great guide (linked above) on determining the best method for you which I recommend taking a look at.
In the meantime, if you can provide more context on your products that will allow me to assist in finding the best shipping method for your business.