Yes, I need to create an ecommerce (app), and I would call a payment service (api) for my application. For the case of the ecommerce administrative panel, would I create a module? I was clear? tnks.
If it is a common payment service API, chances are there is already a package for it. If not, and you need to create something yourself, I suggest you have a look at http://omnipay.thephpleague.com/ and make a custom driver for it. It will save you quite a bit of time. We've made a few over the years as well.
In general, packages are core-extensions, and should be used for backend / server code, while modules are app-extensions, and should be used for user interaction. So yes, an admin panel should be a module.