The best alternatives to Zaprite are InvoiceNeko, Billdogg, and QuickBill. If these 3 options don't work for you, we've listed a few more alternatives below.
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Tanka— Tanka = Slack + Glean, built for fast-growing startups.
Invoice Neko is a self-hosted open sourced invoicing system built on a modern backend with a focus on delivering a good user experience throughout the application