I think the approach would work for what you have described but instead of just having a monogramming option by itself you would just list the various monogramming options. Selecting a value from any of these except for the default of 'none' would ensure that the product had the extra $12 dollars added.
So you would have these three options groups:
monogram thread color
monogram 3 intials
monogram font style
Selecting any value other than 'none' will result in having the price of the item adjusted to what you have specified in the product update page, the regular price plus $12. There are some other ways to do this but they are all custom and are not supported within the standard interfaces of the PowerStore.
