Unsure of the exact plan requirements/mechanisms - I use the basic ecommerce plan and can create a manual order using the Squarespace IOS app on my phone (not in my browser / computer). I can sign in, click "Commerce" and the "+" symbol. I then do what was suggested above, if it's a freebie, and do the 100% discount, which you can do without creating a special discount code since you're logged in as an admin in the app. I do believe this requires a Square account / set up, but this is part of the "unknown exact plan requirements / mechanisms" caveat above.