A Decision Strategy supports decisions so if you have some logic around this it could be used to decide what payment options are available. A flag at product level, or maybe some more customer driven logic. Kicking of the fulfillment process is a different story. In Pega Marketing (not CDH) you could use a hand off shape in an Offer Flow to orchestrate external (or Pega BPM) processes. Without Pega Marketing you need to leverage Pega's business process capabilities more directly.