Malcolm! billing is based on Instances. You can read more about Instances here
We currently bill in UK Pound Sterling and US Dollars.
At the time of writing we accept Visa and Mastercard. We also accept Paypal through which you can use American Express. We don't currently accept Bitcoin :-)
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.