This is an accepted solution.
There wouldn't be a sandbox mode to test purchases on a live site. What I would recommend is placing the password on your store temporarily in Online Store/Preferences during a time where you get the least amount of traffic. From there, you can enable test mode on Shopify Payments or add the bogus gateway to test orders.
Once you have completed your testing, you can disable test mode (or the bogus gateway) and remove the password from your store. You can also edit the password page to notify your customers on why the site is inaccessible and when you expect to be back up and running.
Let me know if you have any questions.