You see the receipt for your paid subscription on your device.

SAU app for Mac / iOS
If you subscribed using the SAU app on your iPhone, iPad or Mac, you can see the receipt on your device or through iTunes. The complete and up-to-date instructions are available through the Apple Support.

SAU app for Android
If you subscribed using the SAU app on your Android phone or tablet, you can see the receipt through your Google Play account. The complete and up-to-date instructions are available through the Google support.

SAU app for Huawei
Subscribing through the Huawei AppGallery is not supported yet.

If you subscribed using the SAU.app web portal, you can see the receipt in your user profile.