Sorry if this has already been discussed, I wasn't sure how to search for it.
Simply put, I want to be able to have a basic (non-cart) registration with an add-on that can change in quantity. People register for a course and then can specify the quantity of the add-on (default being zero), with each add-on being the same price ($15).
I've used dropdowns with a custom field for different priced items or items with different options, but I can't figure out how to make it a number field that will then add on whatever number x $15 to their total fee. Is that possible?