Can have a question before the exercise that has a choice of payment options (and lists the levels of each), and then for your attribute list use a constructed list that adds one of the two attributes based on their choice of payment option. This way only one of these attributes is going to show up in your BYO and beyond. I believe* the non-selected attribute will have a 0 importance in your simulations, but kinda curious myself exactly how that works out.
If your parent attribute list was called "atts," and had 7 total attributes, the constructed list could look like this, where attchoice is the question that ask which payment option to use(upfront = 1, installments = 2:
ADD(atts, 3, 7)