Unfortunately, I have the exact same issue/problem so am NO help to you and found YOUR post because I was looking for the same solution. That being said, have you at least found a way to auto generate the little address label with your customer's address on it? (I've only learned how to copy and paste the addresses so would at least love to find how to auto generate multiples customers address so they would auto print on a small mailing address label.)
That being said, IF you have found a way to generate the address label with no postage on it for multiple orders/customers at one time.....have you considered buying pre-stamped/forever stamp envelopes form the post office? This is what I'm considering doing so I don't have to peel and adhere the postage at least.
I did not find a solution therefore I just went with the $1 solution that Shopify currently offers. Not ideal but has saved me a lot of time with order processing when processing upwards of 20+ orders at a time the Thermal printer is able to spit out my pre postaged and addressed labels 20 at a time.
Shopify offers their First Class Postage Domestic Rate at $1 versus the .55 cents solution offered from USPS. I purchase the postage directly from Shopify and bulk process orders through the native Shopify experience printing out on my Rollo Thermal printer. At this time I only sell within Shopify hope this helps.
Use the Multi Carrier Shipping Label app that automatically imports all your orders along with the customer address. You can then print the labels for all the orders in bulk with a single click. Also, the app offers the same rates, whatever your USPS account offers.
I'm pretty sure you have figured it out by now, but I had the same issue.
Under Settings, shipping, packages, I think you have "Package" as saved packages.
Select "Add Package" and for Package type, make sure to pick "Envelope" or "Soft Package" instead of "Box".
That solved my problem and now shows $1 First class mail as a shipping