Sample credit: when a paid sample order completes, what the customer paid comes back as a single use coupon for the sampled products, limited to their email and valid for a set number of days. Logged in customers get it applied on its own when a sampled product is in the cart, and everyone gets the code by email.
Follow up emails: one or two emails a set number of days after the sample order completes, with a percent or fixed coupon for the sampled products and links back to them. They stop once the customer buys.
