Thanks ChuckMN! I've read on the Internet that a non-profit organization, an accredited educational institution or a government agency do not have to pay those $99. And isn't our book lending a non-profit organisation?

Thanks! There it says: "To join the Apple Developer Program at no cost as an eligible organization, select the option to request a fee waiver when completing enrollment." Unfortunately I can not find the internet address for the enrollment. Please help. Albert

I found the page and found out that for our book lending I would need a DUNS-number. But I can not get one for our book lending. So I have to forget to use the Apple Developer Program.