We have two environments for general use:
https://api.staging.justgiving.com – paired with www.staging.justgiving.com
https://api.justgiving.com – paired with http://www.justgiving.com.
Staging is a live-like environment kept in step with our production releases. Production is the public facing JustGiving website. We strongly suggest you develop and test against staging.
It might be worth noting that https://api.sandbox.justgiving.com is a change from the original https://api-sandbox.justgiving.com (it broke some tools I'd written).