D.C. Mun. Regs. tit. 9, r. 9-403
The Act exempts from the tax the gross receipts from professional, insurance or personal service transactions which involve sales of tangible personal property as inconsequential elements and where no separate charges for such sales of tangible personal property are made.
The phrase "sales as inconsequential elements" shall be deemed to include any sales of tangible personal property made in connection with professional, insurance, or personal service transactions where the sales price of the tangible personal property is less than ten percent (10%) of the amount charged for the services rendered in the transaction.
Gross receipts exempted under this section shall not include the gross receipts from services taxable under § 114(a) of the Act, regardless of whether tangible personal property is involved in rendering the service.
D.C. Mun. Regs. tit. 9, r. 9-403