Well, here's where you need to decide whether you're just willing to let the small things go - i.e. swallow a little loss - for the sake of peace and goodwill.
I think technically you are entitled to demand that she pay return shipping.
But if you can work it out such that she can send you enough to make up for the $5 shipping, I personally would do it. In the spirit of "we're all in this together." You're saying the $5 could be put to another trade, but here you have a mama offering to add more stuff into your package, for that same $5, so in the end isn't it all the same? Especially if the items she is offering are things you are ISO anyway..
This. Is the additional conflict worth $5? Your call...