I am not sure if I completely understand your workflow. Let me point you to the Payment FAQ documentation first, http://forums.adobe.com/docs/DOC-1632, please check the section on how to set your fields as payment fields.
Will you be able to provide an example?
Say if you have a drop down list of "how may t-shirs would you like to order?", 1,2 or 3, then you would set drop-down-field-3 as a purchase field and set the Price to 'fixed' and enter the amount (cost-of-t-shirt times 3) manually.
Hope this helps,