Malcolm! billing is based on Instances. You can read more about Instances here
We currently bill in UK Pound Sterling only so if you are not UK based your debit or credit card provider will convert this into your local currency. In 2019 we do plan to offer billing in US Dollars and Euros.
At the time of writing we accept Visa and Mastercard. American Express should be accepted very soon. We also accept Paypal. We don't currently accept Bitcoin :-)
Micro and Team plans are paid monthly in advance. When you trial ends (or if you end your trial early and remove the restrictions) we'll ask for your card details and every month we will automatically take payment unless you cancel. If your card expires or the payment fails we will send you a number of automated emails to ask you to update your details. If no action is taken after a grace period we will stop serving your content (ie your FAQ's and Workflows) and after a further grace period we will delete your Instance.
Enterprise plans are more bespoke in nature and are negotiated on a per client basis. These can also be paid by credit or debit card but it's more typical for clients to pay quarterly or annually via invoice and bank transfers.