Can I customize the body of the email for payment reminders or just the Subject line? I wanted the system to automatically include the amount, invoice number and due date

Good day, customer-relatio.

I'm here to share some information about template modification.

You can edit the message of the body, though the system is unable to add the date and amount automatically. However, you can add the invoice number.

Let me guide you how:

1. From the Gear icon, select Account and settings.
2. Go to the Sales tab.
3. Click the Reminders section.
4. Click the Insert placeholder drop-down, select Invoice Number.
5. Modify the Email message.
   
6. Click Save and then Done.

Once the system downloads your customer payments, you can match them. Here's an article for the detailed steps: Categorise And Match Online Bank Transactions In QuickBooks Online. 

I'll be right here if you have other concerns with recording customer payments. Have a good one!