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I bought a biannual or annual Premium subscription but it charged me the total up front?

Apple/Google handle all payments and subscriptions, not muzmatch. Monthly subscriptions are charged to your Google/iTunes payment method every month.

Biannual/Annual are charged once at the beginning of the term and renew at the end of the term unless cancelled.

Apple/Google control all payments - not muzmatch, and this is how they treat all biannual/annual subscriptions on ALL apps offered in their Stores.

muzmatch is treated no differently in this regard.