Tag Purchasers Who Use a Coupon?

I would like to tag FluentCart customers who purchase using a coupon (ie: tag them in FluentCRM) but I can't see a way to do that as there is no trigger for coupons. Nor do I see a conditional that would do it.

The best idea I had so far is to create a smartlink that adds the tag when they go to purchase but I am reluctant to do that as I had big problems yesterday with a purchase link in a FluentCRM email promotion.

When they tried to checkout it gave them an "no active cart" error.

Not sure what caused that, maybe link parameters added by FluentCRM or a caching problem (the FluentCart plugin updated just as I sent out my newsletter!)

As a result, I am too scared to use links in the next email (that was a disaster - so many people just gave up in the checkout) and I will just instruct them to go to the shop and add the coupon manually until I can fully test and identify what went wrong.

Anyway...I would love to tag those coupon users.

Any ideas?