For an introduction on BillerBot, please see our BillerBot Overview article.
Using one billing model and set of rates for all your accounts probably won't work too well. BillerBot allows you to fully customize the billing model and rates for each of your accounts. Any trips created for the account will automatically use the customized billing model and rates.
To customize the rate for an account, go to More -> Accounts, and choose the account you wish to customize.
Click 'customize' under the Billing Settings section.
From here, you have the same set of options as you did when configuring the default rates for the organization.