How can I invoke npm on heroku command line (to install bower components)?

You can also setup a postintall command, something like this in your package.json

"dependencies": {
    "bower": "0.6.x"
},
"scripts": {
    "postinstall": "./node_modules/bower/bin/bower install"
}

Then npm install will also install bower dependencies.

Pros : one command to rule them all.

Cons : you unnecessarily embed bower as a dependency.