It would be a better idea for you and the pups if you wouldn't buy from the internet, brokers, mills, etc. Instead, find a reputable breeder, visit the home, see the parents and the babies, read the contract, health reports etc, and THEN decide which baby you want to give a forever home to.
Tinies are cute, no doubt but realistically, they are MUCH more difficult to raise. Mine is a bitty one at just 3 lbs and I don't know if she'll get any bigger but I sure hope she does! She is just so little, so easy to trip over or step on, and I am constantly worried about her getting hurt. They also have more health issues compared to the larger (if 5 lbs is actually considered large here
) dogs might have.
Yes, you may have been duped a bit by her giving you the b.s weight guarantee but you should have known that wasn't possible in the first place. The price was great either way and you have to take your share of the responsibility here. You didn't do your research and you got bent. Lesson learned. (hopefully)
I really hope you don't take this out on the pup and that you can love her and take care of her forever. If not, please rehome her, she doesn't deserve anything but the best!