Can I buy extra tokens?

Yes, on any paid plan, with “Buy Tokens” in Settings or from the plans screen. Each plan buys its own pack: Basic, 250 tokens for $2.49; Standard, 600 for $4.99; Premium, 1,000 for $7.99; and Max, 3,000 for $17.99. They're added as soon as the payment is confirmed, never expire and are spent before your plan tokens. The free trial can't buy packs.