this feature appeared on my account a couple of weeks ago and although i use USPS to ship orders, stamps.com rates are still way cheaper than the rates from inside SQSP.
what would have been a useful feature (at least for me) is if they included a memo feature for when refunding customers (e.g., for shipping excess, etc.), as well as a way to add to an existing order when customers want to add more to their order, and better yet...a manual order creation feature for customers who pay off-site. SQSP doesn't take a cut/commission on sales made on Advanced Commerce accounts so i don't understand why the last two features don't exist when it does on other platforms.