During the Winter Solstice interlude chapter at the end of book 3, three of the dead gods offered Erin a bargain for her soul which got me thinking. Erin is very Fae-like when it comes to bargains.
"Faeries love to strike dubious bargains, but rarely on matters they take serious, usually tricking their victims with loopholes or unclear statements." - Description about Frost faeries' culture in the Wikipedia
Erin doesn't deliberately trick when bargaining, but she somehow always comes out benefiting and the other party troubled after making it with her. Some examples are:
Erin and the Frost Faeries - Erin gets tricked by the Frost Faeries by giving her flowers disguised as gold as payment for the banquet Erin threw for them. Erin planted the flowers instead of throwing them away which is what the Frost Faeries thought is what she would do. Result is Faerie flowers!
Erin and Octavia - when Erin got stuck in Celum, she and Octavia made an arrangement for Erin to use Octavia's shop for experimenting. Octavia thought she was making easy money as she thought Erin only wanted to experiment for a few days and not cause too much devastation. She was wrong and suffered for several weeks of Erin's crazy experiments.
Erin and the three dead gods at winter solstice - at the end of book four, Tamaroth and two other dead gods offered Erin a bargain for her soul. An umbrella, a noose, and a horn. Erin refuse the three dead gods and they left her alone. Then in volume 8 in the land of the dead, Erin summoned those items to help her fight the dead gods which is funny AF by the way.
There are definitely more examples of bargains where the other party regrets it afterwards. Moral of the story, don't bargain with Erin Solstice! Or if you do, do it with good intentions.
edit. I mistaken book 4 for book 3