Hi I'm trying to get a parcel delivered to a post office so that I can collect it but I can't find the option to do so anywhere, even with credit/debit card. 
Contact customer care via live chat.
Hello!
It was down for a moment yesterday as RM had troubles but it should be fine now. Just make sure you are paying by card only, and are using the Royal Mail 24 or 48 tracked services :)