IMPORTANT UPDATE: If you are using the new UI, the "Sandbox test" option (V3) has been replaced with the "Debug Test" mode (V4) -- read more about it in this article. Due to this change, sandbox mode is being retired, after which Blazemeter will no longer offer free Sandbox testing.
The following article thus only applies to customer accounts who have not yet migrated to V4.
BlazeMeter offers unlimited Sandbox testing for all paid plans, which lets you run as many trial tests as you want as we are aware that setting up complex tests usually requires several rounds of debugging your test scripts.
Any test with the field checked for 'Sandbox mode - free low-scale test for debugging purposes' will be considered a 'Sandbox test', and will not charge a credit for the test run.
Please, know that the "Sandbox test" limits up to 20 Threads/Virtual users, up to 20 minutes duration, and running on the console only with 0 engines.