@HiraOmnibus It might be out of revenge, but rather than send an experienced assassin, they send an expendable, inexperienced one to "test the waters." This way, if Carla is caught and fails, then they know that the MC's family aren't pushovers and that her family is probably aware of them, all without losing too many resources. If Carla succeeds, well they got what they wanted and Carla may become a more valuable member they can make use of, if she can escape after the assassination. It's all about profit, really, and if someone found a way to ruin their profit once then that person can do it again, so they want to nip this flower in the bud sooner rather than later. As for how profitable it is to scam wheat farmers, it's probably not just wheat trade they lost out on in the long run. If simple wheat farmers from an insignificant village become literate and can properly see shitty deals for what they are, then so too can other villages they might be currently exploiting, regardless if they be wheat farmers or not.
Of course, this is all assumption and there could be something else entirely at play. Who's to say why someone decides to assassinate another. Could simply that they needed some way to test Clara, and our MC just happened to be someone they could use as a final test before Clara is promoted to a higher position.