How do I add my company stamp / signature to invoices?

Sliptree doesn't currently have a dedicated stamp or signature field on invoices. Modern invoicing — and Estonian, EU, and most international practice — doesn't require one: a properly issued invoice with your company details and VAT number is legally complete without a stamp.

If you really need one for a specific customer or jurisdiction, one possible workaround is to bake the stamp or signature into your logo image — a transparent PNG works best. This is far from ideal (the "logo" then carries information that isn't really a logo), but it does get a visible mark onto every invoice. See Customising invoices and estimates for where to upload the logo.

If what you actually want is the name of the person who issued the invoice to appear on it, that's already supported — turn on Show invoice creator name in the document editor's settings panel.