Look super ****ing embarrassed about it, make the cashier look at you wierd, then exclaim "I'm preop"

Bam, situation diffused. Now the cashier is shittons more embarrassed than you are and you get to troll.

(no offense to anyone who's a preop transsexual intended, ofc)

But seriously, I've purchased them in the past and never made a big deal about it. When I was a teenager buying condoms was awkward, mostly because I had shit for confidence.