when separate books is to be maintained for gratuity
For sales made on or after July 1, 2014, unless a separate amount for tax reimbursement is added to the price, mobile food vendors' sales of taxable items are presumed to be made on a tax-included basis.This presumption does not apply when a mobile food vendor is making sales as a caterer as defined in (i)(1).n optional payment designated as a tip, gratuity, or service charge is not subject to tax. A mandatory payment designated as a tip, gratuity, or service charge is included in taxable gross receipts, even if the amount is subsequently paid by the retailer to employees.