By creating "custom fields" in the "custom data" section through the checkout API, the IDs of the reported apps are converted to lowercase letters.

1. Use the POST Update Custom Fields API;
2. Send an "app" that has an "ID" with an uppercase letter (for example, "customFieldTest");
3. Note in the response of the request that the "ID" was converted to lowercase letters (in the example above, it would be "customfieldtest").

Currently this is an API restriction. It's not possible to get around this scenario. Custom Data Custom Fields must be used with lowercase letters.