It's just me, can I use this for one-time use?

Go right ahead! The signature designer is quite powerful and great for individuals. You can use it for free if you would like. Our plans are geared towards companies and agencies that need to get a whole organization on the same page when it comes to their email signatures.

Will updates be automatically applied to already-created signatures?

Unfortunately not. That's the downside of the simplicity of this way of distributing signatures. When you update a signature template you will need to re-send out the link to your employees to re-generate their signatures and they can copy/paste the signature into their email browser again. Luckily it only takes a few minutes to do.

Why do you charge a monthly fee rather than a one time fee?

For a few reasons, if you are working at a company then employees come and go, and you can continue to send out the link for new employees to generate their signatures at any time. It also enables us to host your uploaded images and keep our servers running.