I agree about /actions, it is unnecessary. However, for these kinds of "action resources", POST is definitely the best-matching request method. It does not mean "create a resource" (although this is one case it is often used for), but "process this request in your custom resource-specific way". The HTTP RFC formulates it like this [1]:
> The POST method requests that the target resource process the representation enclosed in the request according to the resource's own specific semantics.
Given that what an action resource does is, by definition, resource-specific, POST fits perfectly.