I really enjoy this book, and everything by Marcus Zusak. This book is whimsical, and a perfect example of what is known as suspension of disbelief. In other words, you are invited to accept the account you read as real, no matter how bizarre it gets. Suspension of disbelief is a wink and a nod agreement between the author and reader. We know that the story line is implausible, but we also know we are in for a great ride if we ignore that fact.
So, yes, it is pretty unbelievable to accept that a taxi driver is getting playing cards with secret messages with heroic tasks, not to mention actually succeeding in those tasks. At the end, if we play along, we realize that the taxi driver, Ed, is not the messenger but the message and the author, Marcus Zusak, is the messenger.
It’s a brilliant and unusual book. If you want to read Zusak’s masterpiece, pick up The Book Thief sometime.