Each person engaging or continuing in a business subject to the tax levied by this subpart, and each casual sale that is subject to tax, shall add to the sales price and collect from the purchaser the amount due by the taxpayer because of such sale or use. It shall be unlawful for any person subjected to the tax to fail to refuse to add to the sales price and to collect from the purchaser the amount required to be added to the sale pursuant to this subpart. It shall be unlawful for any person subjected 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.