Hi all, thought I'd add an answer for this for those who enrolled via the Developer app on iOS. In that case no invoice was emailed to me and it wasn't visible in the store.apple.com > orders section.
Instead, I followed these steps provided by Apple's customer services:
Open the Settings app.
Tap your name, then tap Media & Purchases. You might be asked to sign in.
Tap Purchase History to see your purchases. To see purchases made more than 90 days ago, tap Last 90 Days and select a different date range.
Find the Apple Developer Program subscription, then tap Total Billed.
Tap Resend to send yourself a new email receipt.
The invoice was then emailed over to me :) Hope that helps.
JamievH
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