Stripe accounts are free and they have a test mode where you can easily create subscriptions and transactions. I think that's beter than a demo.

Yeah, I know, but they used to let you try it without creating an account, and then if you wanted to keep using it you'd just give them an e-mail and a password and you got to keep the API keys and other info.

It wasn't so much a demo as it was an instant trial.

They still let you try it out instantly, they just made the font pretty small on the link to actually do that.

https://manage.stripe.com/register

Note: see the skip this step link