Hi Kevin,
First, I see that her scales are spread apart, but the swelling doesn't appear to have moved caudally toward the vent. I think you still have a bit of a wait
. Second, if the opportunity arises, I'd let this guy know that you understand that sexing can be difficult, and that sometimes accidents happen. You don't hold any ill feelings, but you'd like the resolve the issue and will be posting your experiences (either positive or negative) depending upon the way that this accident is resolved. If this fits your goals, you might mention that you hadn't planned to buy a third snake and don't relish the idea of paying return shipping on the extra snake, but that you're willing to buy the third snake at a reduced rate if he can ensure that it will be a prime looking female.
Just make it clear that you're willing to accept the error but that you still want the female and will be describing how he either rectified the situation or failed to resolve an honest mistake. I wouldn't even bring up the USPS issue... too accusatory.
Rick