GST Rate for oral Hygiene Combo Pack

Rate chargeable would be at the higher rate, i.e. 18% whenever mouthwash is inclusive in combo.

Sir...currently in invoice it is shown as combo pack, what if the pack is combo but in invoice it is shown as separate items..

Would that allow for separate or highest tax rate?

For combo pack it will be highest rate. Otherwise individual rates of each product.

It is a mixed supply, the GST treatment is straightforward under Section 8(b) of the CGST Act, 2017.

Entire combo pack will be taxable at 18% GST
(9% CGST + 9% SGST or 18% IGST)

Even though four items are 5%, because it is a mixed supply sold for a single price, the whole pack will attract 18%.