Wait until the payment has arrived and has enough confirmations (The more confirmations, the safer the transaction is. You need at least 10 confirmations before you can spend the funds.).
Use a @remote-node while downloading the blockchain locally, this will allow you to use Monero immediately and switch to your local node once it's completely synced
This is a POS-like page that merchant can easily use to set the amount of XMR to receive. If the 'Sale tracker' option is enabled, you will see the payments while they arrive in real time in your wallet, along with the number of confirmations.