The structure of the bill is to impose a charge, singular. That element was basic to the architecture of the bill, and to speak in the plural, of “charges”, could introduce ambiguities in the administrative provisions. So by speaking of one charge, the administration can be clear and unambiguous.
In the provisions that follow clause 18, in that regard clause 19 to clause 99 of the bill are those that belong to CRA, the department that administers most taxes in this country. The idea of a single charge was essential to that structure.