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Why do the charities in Production and Sandbox have different ID's? Rephrased, why does the same charity have a different ID in sandbox than it does in production.
Although sandbox functions the same way as production, they use different data sources and thus they may have data disparity from time to time. If you would like to only test using a certain charity you can get the charity ID from sandbox using the following request.
<?xml version="1.0" encoding="utf-8"?>
<CharityName>Hope For Healing.Org</CharityName> <!--specify a charity query, exact names do not need to match -->
In the response you'll get the charity ID of charity with respect to it's environment.
Also, it can be quite helpful to remember that an exact name need not be specified in the charity name field of the request. Any query can be entered and the response will include several charities that match the query.