Sorry for answering so late. Yes, just mark the invoice as paid or something. Your have to have the invoice in the system though as far as I know.

I use InvoiceNinja for what seems to be a very similar use case. After a doable learning curve I really like it. You can install it on bare metal or use docker.

Hetzner has Nextcloud in their your-storageshare.de offer. It’s a hosted instance which you are admin of. About 5 Euros a month for 1 TB of storage for the smallest tier. This was the way I chose after years of self hosting. Your-storageshare is awesome and I don’t get tired of recommending it. Maybe I should ask for a commission at some point…