The trouble ends here, @PianoGirl. I will also feel fulfilled if you can successfully send your invoice receipt again using QuickBooks Self-Employed (QBSE). 

Let me discuss some details on how you can do this.

In QBSE, you simply need to select the Send option from the dropdown selection of a paid invoice to generate a payment receipt and email it to your customer. Please see the screenshot below: 

Since you're having trouble performing this task, we can start a troubleshooting steps to isolate the issue. Let's begin with accessing your account using an incognito mode as an initial fix. Below are the key board shortcuts that you can use depending on your browser type:

• Google Chrome: Ctrl + Shift + N.
• Microsoft Edge: Ctrl + Shift + N.
• Mozilla Firefox: Ctrl + Shift + P.
• Safari: Command + Option + P.

When logged in follow these steps:

1. Go to the Invoice menu and look for the paid invoice.
2. Next, click the drop-down arrow beside the transaction and choose Send.
3. There will be a pop up page that will allows you to preview the receipt before emailing it.
4. Simply click the View receipt button if you want to check the form. 
5. Once done, select the Send option to email it to your customer.

If you successfully sent the receipt, I recommend clearing the cache of your regular browser to eliminate the old and damaged files. If you're unable to do so, I suggest using another supported browser or device. Maybe that the current one you're using has temporary issue with QBSE. 

Customizing your invoice form is easy as one two three in QBSE. This process will further help you express your business identity. To get started, just click the Customize button on the invoice page. 

I'm always glad to provide additional help if you have any questions about the steps above. Don't hesitate to leave a comment below and I'll get back here in this thread as soon as possible. Have a blissful day ahead. Keep safe.