How to Email Quotes to Sub-Customers (about 15 of them) and Email Invoices to the Main Customer?

Welcome to the Community, ciaran.

I'm here to provide instructions on how to ensure that sub-customers receive all quotes and the parent customer receives all invoices.

But before that, when creating a Sub-customer, the Bill parent customer option must be ticked. Here's how:

1. Go to Sales and then select Customers.
2. Click the Sub-customer name, and then select Edit.
3. Tick the Bill parent customer.
4. Click Save.

After making the Sub-customer, we can proceed with making the Quotes. 

When creating an invoice, you can select the parent customer to whom the invoice will be sent via email. Here's how:

1. Click the + New and then select Invoice.
2. In the Customer, choose the parent customer name.
3. In the right corner, there's a quote that will be displayed. You can select the estimate made by your sub-customer.
4. Click Add and then Save and send.

You can also check out this article about receiving and tracking customer remittances in QuickBooks Online: Record invoice payments in QuickBooks Online. 

Should you need additional assistance with your customers, feel free to include them in the comments below. I'll be around to lend a hand. Stay safe!