I send a lot of my products by regular letter mail - I have a large selection of decals and free shipping, so this is a regularly used and vital workflow. I also need my customers to be notified when their product ships, and this is not available in your system. I need to actually go and buy my postage outside your system and manually mark things as shipped. And my customer will not know their product is on its way!
It is one reason I may need to return to using ShipStation again, because they offer this dimension.
Please update your API with USPS to support this dimension! This is 1/3 of the products I sell.