I can see that is possible to put on the website the IOTA address (with Tangle announcemement) for donations. So it means it is possible to multiple pays on the same address.
And my question is: where (like on blockchain) can I find all these transactions listed of the donation address?
And is there any API to get (ie. to GoogleSheet) the transaction result from this donation address?
You can receive as much as you want to an address, but after spending once from the address, don’t spend again from it. To track your transactions you can check out https://steemit.com/iota/@steemhoops99/iota-cool-useful-tool-to-track-your-transactions or use the tangle explorer http://www.iota.tips/ it works a bit different than on blockchain. About the API i would go to the official slack. You will get insight help with that very easily:) Still making a guide for the tangle explorer so stay put for that:)