Is there a facility on Quickbooks for a customer to sign an estimate online?

Hi Kate39, thanks for your post - we don't currently have a feature to enter a signature on estimate forms. We'd be grateful if you could suggest this as feedback by clicking the company cog > feedback in your account - this way it'll be submitted to our developers who regularly review and implement changes based on this. 🙂

@kate39 

You can utilize a free invoicing app with that feature to integrate with QBO. Another option, if you are running a service company, you can utilize a proposal management app.