Ok, so with this technique, i'm now at 434 hearts-

Buy hearts, but don't just tap one time, spam the button and that will buy a lot more hearts than the supposed number of hearts you paid

And I have a theory... I think that you'll receive a number of hearts equivalent to the number of times you clicked before the transaction really effectues. So just click fast or use an auto-clicker idk

NOTE THAT THIS TECHNIQUE WILL TAKE OFF ALL YOUR COINS