What Breed Is My Dog?

I bought my puppy through an online shop, and it cost $54. The seller said it was a St. Bernard crossbred with a Japanese Spitz. She's 3 months old now, but her looks don't look like a St. Bernard or a Japanese Spitz. She has a medium-sized paw and ear and she is heavy for her age. Out of curiosity I searched for an answer and I end up on this page. I am hoping someone could help me find out my dog's real breed. Thank you.

My puppy is a smart dog, very sweet, and playful inside our home.