PubHTML5 offers users 4 different packages including Free, Pro, Platinum and Enterprise package. When you want to pay monthly for this software, you can choose the Pro, Platinum or even the Enterprise package. When you choose different packages, you can have different privileges at the same time(For more info: http://pubhtml5.com/pricing.php).
Now Pro package supports monthly payment. If you don't want to use the service, you can cancel it anytime.
Platinum plan can only be billed annually.