where am i going to pay for the 50GB icloud storage?

Upgrade your iCloud storage plan -

https://support.apple.com/en-us/HT201318

You pay by using the payment method that you have on file associated with your Apple ID. If you use a credit or debit card, that will be charged. You can also redeem Apple Gifts cards to pay for iCloud storage.

Edit - my apologies, Phil. I wasn’t paying attention and didn’t see your post.

iCloud storage is automatically charged to your Apple ID's payment method or iTunes gift card credit if there is any.

No worries Demo. It happens. Always good to have confirmation.