I agree with gnoams, Dave Justus, and Chengar Qordath. This isn't abusive, but guerrilla tactics can be frustrating. Heck, they're designed to be frustrating in order to cause the enemy to make mistakes or give up. Such encounters are best used sparingly or they'll get in the way of the group's fun.
Having two such encounters in the same session was overdoing it.
As for what next, dropped papers are a good way to clue players in to what to expect. Then have a chat with them. I'd explain what you were trying to do, apologize for taking it too far, and indicate that you have faith in their ability to overcome this challenge, especially if they change up their own tactics. Remind the druid and shaman in particular that they have access to their entire spell lists, which include good spells for detecting or setting ambushes. The druid may also want to consider wild shape forms that have benefits other than just raw combat power, and everyone should consider mundane actions such as concealing or fortifying their campsite.
There was but one such strained encounter over long terrain distance. 2nd encounter was resolved normally per PF rules. Perhaps I didn't clarify it enough in my first post. My apologies.
Your suggestion is what I had in mind. I'll suggest several helpful spells and ideas. It's not the first time they will encounter someone ambushing them.
I did misinterpret that, sorry. However, closing to melee in the second fight was apparently not enough for the players to feel they had worked off their initial frustration - the ambush and the two surviving mercenaries' successful withdrawal may have resulted in them feeling that even if they'd won they had been made fools of.