Regarding the issue, the FIWARE Lab is an experimentation environment and thus, the payment gateway is disconnected in order to allow users to experiment with the platform without being charged.

On a real environment, a modal is displayed informing the user that she is going to be redirected, then the user is redirected to PayPal, and when she approves the charge, the user is redirected again to the Store.

yes, this is an issue of the Store.

Moreover, note that the integration of the CKAN instance in the FIWARE Lab with the Store (often referred as the Data Market) is not overriding open data features you are providing. This is something that according to the DOW is being provided by the Apps Chapter is response to some requirements of accounting and access control made by some institutions before publishing their data.

Is this an issue with CKAN? Is this not an issue with the store?

The dataset is purchased outside of CKAN, e.g. for the Madrid Transport dataset, one buys that here: https://store.lab.fi-ware.org/offering/sergio-garcia/Madrid%20Transport%20Evolution/1.0

I also wonder if this is within the scope of the project (at least the Open Knowledge end of it). The DOW talks about managing and publishing Open Data: "The capacity to manage and publish Open Data from internal or external data sources and to use this Open Data to feed or enrich the rest of the GEs in the Chapter." which according to the open definition should be free of charge or at reproduction cost (free for digital assets): "The work shall be available as a whole and at no more than a reasonable one-time reproduction cost, preferably downloadable via the Internet without charge."