If the evar is set as conversion syntax, you would have to make sure all the below mentioned actions are happening on the same hit:
- The merchandising eVar is set with a value.
- The products variable is defined with a value.
- A binding event is set.
Once all these actions are there in the same hit, Instance will be counted for the evar value otherwise it will go under None/Unspecified.
If you are passing the merchandising evar in product string than I guess it should be set as Product syntax in which you do not need to set any binding event. The success event should fire in the same hit in which product syntax is fired to make sure evar gets bound with event.