Can I buy the internet package with installation modem directly at the KPN store or can it only be ordered online?

Best is to go to https://www.kpn.com/shop/internet-tv/internet?icid=toptaak2internet and select the things you want. In some cases we can deliver within the same week but this depends on a lot of factors like if there should be any work done in the infrastructure or the current capacity of mechanics if needed. But I cannot see into your situation so I don't know if this would be possible for your address.

Last week I had one installation that was ordered the day before so it can go really quickly