use eCart for this.
create a new column in the cart object for credit option.
in the add to cart button, set the price column to be editable from a select list. set the credit option column to updateble from a hidden form element.
after the add to cart button is added and has created the select list and hidden element for you, populate the select list with the labels and values
Now, you will need to add an onChange event to the select list to populate the hidden form element with the selected label. make note of:
1) The form name
2) The select list name
3) the hidden element name
the onchange script will look like:
onChange="document.formName.hiddenElementName.value = document.formName.selectName[document.formName.selectName.selectedIndex].text"