Custom Integration: Access Token ONLY Works in Production Env!

(I'm using a code block for the body of this message because the GUI keeps interpreting my underscores as markdown Italics. You should fix that.)

The documentation tells me I can use my access token
with the prefix "sand_" rather than "prod_" for integration testing.

When I try that, both in the platform I'm using, and in the EasyShip API Documentation
site's "Try It" feature, authorization fails. Any API call I try, the "prod_" token works
and the "sand_" token does NOT.

I reached out to support, and they told me to come here. I found this other thread
describing the same behavior, and the responses to it were wildly unhelpful:
https://developers.easyship.com/discuss/634695206698f9002fc13d27

The thing I need to test right now is the creation of a labe
(via https://api.easyship.com/2023-01/labels) and I'm not willing to actually pay
for my test labels. This needs to be resolved ASAP!