The short answer is (in my experience) you're going to have continued challenges doing this. Marketo's Analytics, and its program architecture and philosophy, are based on the concept of multiple marketing initiatives touching a lead over time and influencing an eventual purchase.
Marketo is not set up, out of the box, to track short, repeat purchases that are more typical in a B2C environment and some B2B environments.
My most recent company was B2B but functioned in a similar way as what you are describing -- direct response style emails that could generate quick purchases. The questions being asked were the same.
Check out my answer for this thread for how we solved it: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Q35DAAS
I should also say you need to encode whatever attribution logic you have into the trigger that changes the program status.
Off the top of my head, I could think of some alternate ways of potentially doing this, perhaps having multiple revenue cycle models for each type of repeat purchase. But it is really square peg round hole and would get complicated.
If anyone else has done cool things making Marketo's analytics work well for this type of business I am interested to hear about it too.