Sometimes Apple may take a little time to process your payment and activate the subscription. In the meantime, please try these steps:
Open Settings in the app
Tap Restore In-App Purchase
Restart your device
Try sending the invoice again
If the issue continues, please make sure you’re signed in using the same login method you originally used for Invoice Maker (Apple, Google, or email). Your subscription is linked to the first account connected to the app.
If you still need help, please contact our Support team.