Wow, shopify has a $20b market cap, and they cant figure out how to handle multiple boxes for one single UPS shipment for one order?
By shipping partials, 3 times to make up a 3 box shipment, freight is 30% higher than if it were a multi box ups shipment.
In addition to that, instead of 1 of 3, 2 of 3, 3 of 3 with one signature required, we are also charged $5.35 3 times for signature confirmation for each box, where in a multi box shipment, it would be one signature for all 3 boxes.
Since Shopify needs to handle common scenarios for majority of storeowners, hence makes sense to stick with pure weight based calculation of shipping rates. However, cases like yours can also be fulfilled very easily with the help of Multi carrier shipping label app. With this app, you can define a threshold value of weight beyond which a new box is selected. For example, if the order has 30 lbs as the weight and you have define a maximum weight of 10lbs each box can carry, then it is this order will be packed in 3 boxes. Moreover, the labels will be shown as 1 of 3, 2 of 3, 3 of 3 and finally the signature option will also be applicable for one shipment as a whole and not for 3 boxes separately.