Thank you Qualiteam!
I would like to add that Roman
Nikiforov (from Helpdesk) suggest's:
"The 'price modifiers' is the type of a product options which changes the 'parent' product type
The product variant -
is a separate product with its own price.
For more detailed information please refer to the following article:
http://help.qtmsoft.com/index.php?title=X-Cart:Product_Options
and see the following chapter:
http://help.qtmsoft.com/index.php?title=X-Cart:Product_Options#Managing_Product_Variants"
So, I understand that to mean setting up my products (single units and cases) in this scenario as a variant is more appropriate than an price modifier- I suppose? Also, if it is a variant, the price is not displayed next to the product in the drop down!