Right now in QuickBooks Online, there's no Sales Order feature. However, you can create an Estimate as a replacement. An estimate is a non-posting transaction that can be turned into an invoice at a later time. When the customer gets the invoice, this can act as the sales order.
On the other hand, a Purchase Order (PO) is a non-posting transaction that you can send to a vendor to state an intent to buy products or services.
As for the process of recording the advance payment made from a customer, you can use the Receive Payment feature.
Here's what you'll need to do:
Set up the customer.
In the left menu, click the + New icon.
Choose Receive Payment.
Select the customer's name using the drop-down arrow.
Under the Account column, choose the invoice and enter the amount of payment received.
Hit Save and close.
For the accounts to use, I recommend checking with your account to ensure that this transaction is properly tracked.
Additionally, I want you to get the most out of QuickBooks for your business. You can check some articles that are designed to help you get acclimated with the software. They can be accessed at the following link: QuickBooks Tutorials.
As always, you can get back to me if you have any additional questions. I'm here for you. Have a great day!