You had me rolling with the line in chapter one: What if I use my left hand? That was priceless!
I like the slow build to the sex scene. Though this is a PWP, characterization isn't sacrificed, and I appreciate that.
The only thing I have any issue with is the labeling of the speaking character at the top of the chapter; it's always been a pet peeve of mine. I understand why in this case you felt it was necessary, as you switch perspectives in the last chapter. I have always thought, though, that if the speaking voice reflected the character well enough, the notation was unnecessary. As you make it clear that the Omake is not in Renji's voice, I don't think it's a necessary notation.
To the sex itself, I like how you include enough emotion on both their parts to be seen. The rendering was very much as though we were included in Renji's experience.
Thank you for sharing this short story. It was an amusing read.