Buying a single conversation
Buy just one, right from the chat, and it opens as soon as you've paid.
If you have a mutual yes but no conversations in your balance, you can buy a single one from that chat.
Once the payment goes through, the conversation opens for you automatically and you're taken straight into it. If it can't open any more (for example, they opened it first or it expired), the conversation stays in your balance for next time.