Triggering events when customer spend passes thresholds

Hi,

I’m trying to set up a flow for the following use case:

“Every time a customer’s total spend passes a $200 threshold, send them a promo code with a $10 discount”

Our system uses the Klaviyo API to send subscribers/orders, and has a end-point to trigger the promo code creation (and trigger another flow to send an email).

How would you go about tracking how much a customer has spent and when they’d been sent previous promos?

I’m trying to do this using a “Dollars Since Last Promo” Subscriber property and increasing this by order totals, then decreasing it when an promo is sent. However, it seems (unless I’m mistaken) that Flows cannot modify a subscriber property based on its existing value or an event property.

Can you think of a way to achieve this outcome?
 

Thanks!