If you have a credit on your account, it'll automatically be redeemed on your next job. 

Once the job is completed, you'll see the account credit applied after the tax inclusive amount, just like a gift card would be. Tax is charged on the original amount of the job, and not after the credit is applied since it's Jiffy paying the the credit amount, not the Pro. The Pro then receives their normal portion of the total job, irregardless of any account credits.