I can download extras in Modx 2.4, but not all of them. I can find and download Quip, but it is not possible to find Shopkeeper or getProducts via Modx 2.4 package manager.
When developers upload their extras they can specify the version their extra is good for. Many will say "up to 2.2.x", but the extra works just fine on 2.3 and 2.4. In such a case, you'll need to go to the Extras repository, download the extra, then choose to upload it from the install button's options menu.
Of course, there may well be some extras that won't work on 2.4, but it's usually worth a try anyway.
you'll need to go to the Extras repository, download the extra, then choose to upload it from the install button's options menu.
Do I understand correctly that if I install the extra manually I will not be able to get information on extra's updates? Thus in future I have to reinstall it from modx repo?
You can fake it if the extra is in fact in the MODX repository. There are two relevant tables in the database, transport_providers and transport_packages. The MODX repository should be #1 in the transport_providers table. The 'provider' field in the transport_packages table will have 0 if you installed it locally. Change that to 1 and now it will have the Check for Updates button, and will check the indicated repository (#1 for the MODX repository).
Be careful to make sure that the extra really is in the MODX repository. I don't know what would happen if it isn't; I'd really rather not know.