To my knowledge you have to put endergirl into the package box boat and make her cum until she teleports, but I didn't manage to get it, maybe it's a random chance that she will teleport to the place where you get the achievement
Viewing post in LCPT 0.2.999 is released! comments
Nope! I'm afraid that's irrelevant to this "Return to Sender" achievement.
But instead, it is related indeed to the "No-Clip" achiev. To get this - you gotta keep shoving those Ender Pearls repeatedly into Ender-Gal's bum-hole (since yes, the random chance can be from the 1st to 5th time), until it yeets you both to the Backroom.
To get that "RtS" badge tho, you just need to f*ck Endy' (while she's in that Parcel Box) and make her cum/teleport until you both find yourself in the Parcel Warehouse.