Thanks for the update and the additional information you've provided, @mikeotaylor. I'm here to give further details about the option of sending payment receipts in QuickBooks Self-Employed (QBSE). 

Currently, there's an ongoing investigation about the disappearance of the Send receipt button on both iOS/Android mobile apps. The recommended workaround for this is to open your account via web browser and use the Send option in sending an acknowledgment to your client for a paid invoice. Here's how:

1. Open your QBSE account and click the Invoices menu.
2. Next, look for the paid invoice you want to send a receipt for.
3. Then, select the drop-down arrow under Action and click Send.
4. Review your client details and once ready select the Send button. 

Then, I recommend reaching out to our Customer Care team to link your account to the said investigation. This way, you'll be updated about the issue and get notified once it is resolved. I'll guide you on how:

1. In your QBSE account, select the Help icon.
2. Next, click the Contact Us button.
3. Enter the details of your concern and select Continue.
4. Then, choose either Chat with us or Have us call you.

Let me know how this goes and feel free to add a reply below. I'm always ready to provide answers if you have additional questions about sending payment receipts in QBO. Have a good one and keep safe.