Can I buy extra tokens?

Yes, on any paid plan, with “Buy Tokens” in Settings or from the plans screen. You choose how many with a slider, from 500 to 20,000, and see the price before you pay. Basic and Standard pay their plan's rate (Basic $10.00 per 1,000 tokens; Standard down to $8.35); on Premium and Max the price per token goes down with the amount: Premium from $8.00 to $7.05 per 1,000, and Max from $8.00 to $7.00. Every price is rounded up: to the next 50 cents below 5,000 tokens ($5.00, $8.50…) and to the next whole dollar from 5,000, so on small amounts a token costs a bit more. They're added as soon as the payment is confirmed, don't expire and are kept apart: your plan's tokens are used first. The free trial can't buy extra tokens.