Recently I had a friend gift me nitro about a month ago and was nearing the last few days of that subscription. During the grace period however, my friend sent me another subscription for 1 more month, but as it turns out, this subscription was not redeemed immediately and was instead stored as 'credit' that could be used when attempting to make a purchase for Nitro after inputting credentials for it.
My suggestion is that we should be able to activate Nitro Credit manually if it's being stored onto our accounts, without the need to input any sort of credentials to activate it. I myself and possibly tons of other people do not have access to a bank account/PayPal needed in order to activate this credit. As such I would find it extremely convenient if I could just activate the stored credit manually without having to worry about needing credentials in order to use it at all.