I applied to several post-grad programs at Australian Universities. I got an offer from one (University A) very quickly, conditional on completion of my undergraduate degree. It had a lapse date of January 1. During the Christmas break I emailed University A letting them know that I had completed my undergraduate degree but would not be able to get a transcript to them before January 1. I said "Is there anything I can send in the meantime [while waiting for official transcript] that would be sufficient to prove I have met the conditions of the offer and can accept it?" Since then I got another offer from a different university (University B) that suits me better and I plan to accept this offer.
Today (January 6) I finally got a reply from University A saying "I can confirm you have successfully accepted your offer...". I realize I may have implied that I was going to accept the offer in my email, but the situation changed and I no longer want to accept the offer from University A, since University B is a much better option for me for various reasons.
How can I professionally and politely decline the offer to University A?