I have a video of myself doing this here:
https://www.youtube.com/watch?v=OcdRJwFa4Hk
Essentially you have to take one of the bags (either the herbs or the bag of sulfur) and try to drop it near one of the bags on the ground*. It'll open up the trade window and you can put the second one there as well. Make sure your FR is high!
* you should be clicking on the ground, not on the bags. Those are just a visual cue.