First of all, I just finished digging up my garden and applying compost to it. So how far will you travel to give a massage?
Next you would want to create a Variable price product and enable the variable name and variable sku fields.
Another option might be the Order API (found under Merchandising) and create a script that receives the parameters pass to it about the order and then use that information similar to what I described above. You could either use the OrderAPi too replace the thankyou screen or you could use it to send a separate email to the shopper with the "coupon" Documentation on Order API can be found at http://shopsite.com/help/11.0/en-US/sc/pro/order.api.html