When you add/edit a product in VirtueMart, make sure to always set two values:

1. The product's tax category.
   e.g. Untaxed (0%) or 10% tax (GST).
2. The product's weight.

If the product has a weight of zero, VirtueMart will detect that the product is a "Virtual Item".
By default, we always set "Virtual Items" to be taxed (at 10%)... regardless of the selected tax category.

If you are not aware of the product's weight, or if your are offering "Flat Rate" shipping ... just assign each product a weight of "1".

FYI:

• The most noticeable symptom of not setting the tax category to 10% tax, is that the "Product Price (Net)" & "Product Price (Gross)" will update to be exactly the same values.
  When the tax category is set to "10% tax", then there will be a 10% difference between the Net & Gross.