Thanks for the responses. I have been e-mailing with Planet X, and they have been promptly replying. However, based on the non-committal language they are using, it appears that they are not going to guarantee a refund or exchange until their warranty team inspects it, even after I sent pictures. Based on some of the comments here, I am concerned about getting a run-around and not having much recourse being in the U.S. Also, CX training is going to start in August, and I would like to have the bike built up by then.
I have not tightened the seatpost collar yet, so the seat tube is in original condition.
I will measure the tube tonight, but I don’t think the entire tube is oversized, as the seatpost collar fit fine.
Wife says I should just eat the cost of my undersized seatpost and buy a new one that fits rather than dicking around with a return/exchange.
I am thinking that may be the least stressful means of fixing this.