If you have a paid plan with Freshbooks, then you can access the Freshbooks developer portal without an additional cost. If you do not have a plan with Freshbooks, you can sign up for a 30-day free trial.
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Even after the trial period ends in 30 days, you will still have access to the Freshbooks developer portal.

You can sign up for a 30-day free trial with Freshbooks here and you can access the Freshbooks developer portal here.
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Please see here for more information on integrating Freshbooks with Railz.