You awake to the sensation of her breath entering you again, maybe minutes, maybe years later. As she ties you off, you think - balloons are ephemeral. People pop them and throw them out without a second thought; hell, when's the last time you even saw someone untie a balloon, you lucky (sum)bitch? In short, if you let her inflate and deflate you as she wishes, before long it'll be the last time.
She puts you between her thighs, rubbing you up against her bare cunt. You feel the pressure from her thighs, and you think about how easily you could pop, how even if her crazed intent really is to keep you indefinitely, she's sure to pop you using you this way. You know what you have to do... and given her proclivities, you'll be doing her a favor.
"Become a balloon," you think, just like before. "Megan, become a balloon. Become a balloon. Megan. Balloon. Megan. Balloon..."
A shriek. Megan's skin, just like yours did, changes, but pink rather than blue. The feeling of your "skin" against hers goes from yielding flesh, polluted by sweat and mucus, to the pure squeaking of rubber. She begins to giggle, and this giggle becomes a terrified, hysterical cackle, a terrible synthesis between her giggle and her shriek. Her legs lose their definition, and you roll away; she rounds out and floats up to the ceiling.
You change your buoyancy all but accidentally, floating up to get your bearings. You're not in the store anymore, but in her apartment. There are fragments of popped balloons all over, ground into the carpet. You quietly thank whatever god has brought you here for your decision not to wait.
"I want to be humanoid. I want to be humanoid. I want to be humanoid."
Arms, legs, and a head extend from your round form. Hands and feet begin to take shape as what's left of the original balloon reshapes itself into a female torso - you couldn't say why female, but fortunately, no one's around to ask - and the knot grows into a permanently erect 20 cm cock. You float down to the ground, taking a few cautious steps, and look up at Megan.