Hello Mileage, 

Thanks for being a part of the QuickBooks Community. I hear what you're saying about saving time. Luckily, QuickBooks Online is designed to make basic bookkeeping tasks easy so that you can get back to doing what you love. Let me help you out with your Accounts Receivable transactions as well as applying customer payments.

When it comes to adjusting the balance of your Accounts Receivable without entering individual invoices for your customers, the best method would be to enter a Journal Entry. Here's a great resource on how to Create a journal entry in QuickBooks Online. Create a customer to represent your monthly sales that you can use in the Name column, and ensure that you select the correct tax codes. While this method will be effective in moving money from one account to the other, when it comes to these transactions it is always best to consult an accountant. 

If you're already working with an accountant, why not invite them to work on your books? Navigate to the My Accountant tab in QuickBooks Online, fill in their email address, then hit Invite. If don't have an accountant yet, the Find a pro to help button will point you in the direction of lots of great QuickBooks savvy accountants that are happy to assist you in making the best bookkeeping decisions for your business. 

Next up - applying customer payments! When your customer is paying you later, the best way to record the transaction is by entering an Invoice as you've been doing, then following up with Receive payment once you've gotten their payment. You can do this from the + New button or from the Sales tab. If you would prefer to enter these two steps in one transaction, you can opt to use a Sales receipt. A sales receipt is typically used when a customer pays you on the spot, but it does save you a step in the bookkeeping process. If you'd prefer to just enter the payment from your customer, this might work for you. 

Don't hesitate to reach out if there's anything else you need support with. Have a great day!