A Math Riddle: But the math does not add up

Grandpa had a lot of different books of stamps on his desk. He was counting them.

I was curious. “What are all those stamps Grandpa?”

“I just got back from the Post Office son and bought a bunch of different stamps! All books.”

“Yea? What did you get?” Not sure why I asked.

“Well, let’s see. Two books of International stamps. One book of 75 cent stamps.
And I got – 20 cent stamps, 50 cent stamps and also 55 cent stamps. That is it!”

“Lot of books, Grandpa.”

“Yes, son. I spent 50 dollars in total. Let’s see. Wow. I have a total of 13 books of various stamps.
So do your math and tell me son, how many books from each stamp category do I have? Here is the pricing. I only bought stamp books; no individual stamps”

I did the math. It did not add up.

“Are you sure you counted the books right, Grandpa?”

“Yes, absolutely!”

“No other stamps than what you told me?”

“Nope. Only full books, 10 stamps in each book.”

Hmmm. Why was he right?

Postage Pricing
1 Book = 10 Stamps

2 cent stamp	20 cents per book
5 cent stamp	50 cents per book
10 cent stamp	$1.00 per book
20 cent stamp	$2.00 per book
50 cent stamp	$5.00 per book
55 cent stamp	$5.50 per book
75 cent stamp	$7.50 per book
International stamp	$12.00 per book