Problem: You are trying to test a Donation form on Soapbox, and are using Authorize.net as your payment processor. You have the Donations form set to Test Mode per the instructions here but when you submit the payment, you get the following error: "Payment Error: (13) - The merchant login ID or password is invalid or the account is inactive."
What's Going On?: When you set the Authorize.net payment gateway to test mode, it actually uses the sandbox (a test environment) in Authorize.net. The sandbox environment has a completely different set of credentials.
Solution: If you want to make test transactions with a test credit card against the Authorize.net sandbox (test environment), you'll just need to enter in the sandbox credentials for Authorize.net into the "Payment Gateway - Authorize.net" plugin (Extensions -> Plugin Manager) and set "Test Mode" to "Yes."
Note that this will put all site transactions into test mode.