Each person engaging or continuing in a business subject to the tax levied by this subpart, shall add to the sales price or admission fee and collect from the purchaser or the person paying the admission fee the amount due by the taxpayer because of the sale or admission. It shall be unlawful for any person subject to the tax to fail or refuse to add to the sales price or admission fee and to collect from the purchaser or person paying the admission fee the amount required to be added to the sale or admission price. It shall be unlawful for any person subject to the tax levied by this subpart to refund or offer to refund all or any part of the amount collected or to absorb or advertise directly or indirectly the absorption or refund of any portion of the tax.