If you're subscribed through Google Play Store or iTunes, head to your Play Store/App Store subscription settings to view your invoices.
For all other subscriptions, view your invoices right in your app!
On Android, tap the Menu button on the top left-hand side of your screen, Settings > Invoices.
On iOS, tap the My Music icon in the bottom right-hand corner > Settings wheel in the top left-hand corner > Pro Options > Invoices.