Thank you both for your replies.
Neither of these amendments is working for me. It still comes up blank on the shipping quotes. Perhaps I have something else to change in my settings as well--but I was sure I'd done everything. I'm double-checking now; any suggestions are welcome!
I wasn't expecting the solution to this problem to be in the USPS module, but rather the shipping module itself. Particularly considering the comment below from the other thread on this subject:
I have exactly the same issue, even with just flatrate delivery enabled. I've disabled all of the shipping related modules which does at least enable test transactions to be made, but I need to find a solution before the site can go live.
I also have the "Weight Quote" module enabled, and it's not showing anything either.