In my case, it was a size S. My educated guess is that their production run priority usually goes M > L > S > XS/XL.
I’ll admit I might have jumped the gun a bit—I wasn't fully aware of how PayPal disputes affect OEM manufacturers behind the scenes. In my defense, waiting itself isn't the issue; I don't mind a long lead time at all. What frustrated me was having the estimated date pushed back week by week. If they had told me upfront that it would take an extra 6 weeks, that wouldn't have been a problem.
Ultimately, it boils down to poor communication.