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In it, it shows the Spotify username you paid for:
Logout of the app, and be sure to use it to login to your account. It's the one with the Premium features.
If you didn't get any email receipt, sometimes the amount gets 'on hold' even if the charge doesn't go through. Normally the amount gets back to your account a day or two later.
In any case, you should contact customer support through this link:
They'll help you out. They take, at the most, 24 hrs to get back to you.
Don't forget to check your spam folder for responses as they may end up there.
If you get an automated email directing you to the community or help pages, you need to reply directly to email@example.com and one of the customer services agents will get back to you.