need to cancel a pending order that isn't going through

I think you are fine and shouldn’t worry. I had UPS lose my phone delivery once and TMobile had to send me a replacement. The original lost order still showed pending for about 6 months before it stopped showing up in my orders. Also note that Tmobile does not just sell phones without plans so your new phones are already tied to their respective lines so Tmobile cannot just bill you for another phone on the same lines.