Tap the Pay button in the message (or follow the link starts with t.me/send?=IV...), then tap a Start button, choose a cryptocurrency and tap Pay Now. After that, coins will be instantly transferred to the recipient’s wallet.

Any invoices you open are added to the list of unpaid invoices where you can view and paid them later if you wish. To find your unpaid invoices, go to Invoices → Unpaid Invoices.