Christine here from the Shopify Guru team - thanks for reaching out!
You are able to print shipping labels using USPS currently in your Shopify store admin during the fulfilment process. If you are using another carrier you are able to include the tracking information for the order.
If you would like to print a shipping label using USPS that is not associated with an existing order, you could create a draft order. To do this, you would create a product called "Shipping Label" and then you would associate a weight to the product based on the price of the label required. In the draft order you would also indicate the customer for the shipping label. Lastly, you would then mark the draft order as paid in which case it would become an order that you could then fulfil and print a label with USPS.
Hope that helps! Let us know if you need anything else. We are just a click away :)
I just needed to do this and found this thread. I was able to do so by creating a custom item at $0.00 that does not accrue taxes, and requires shipping, and I also modified the shipping to be free. Then I chose my customer. I am sending a pair of earrings to someone for free as part of a giveaway, so this seems to have worked well.
The system still recorded the order, but it was at $0.00, and then allowed me to purchase my shipping label per usual, but was "free" to the customer, therefore only charging my account at the discounted Shopify Shipping rate.
Hope that this helps someone else!