The truth about what you are giving as bonuses, either as a vendor or an affiliate, comes down to one main thing.
There is a right way and a wrong way to do it. A lot of bonuses are just stuff that does not relate to the product being sold, it is just old stuff the person has sitting around.and means nothing. This is the wrong way to do it, though it is very common.
The right way is to have the bonuses to match and help improve on the product that is being sold.
A lot of the time I will make a new product to match the one I am selling, be it a book explaining things related, or a quick bit of software, a plugin, or anything that provides extreme value to the customer.
The customer will then wait and explore what you are promoting next time and become a repeat buyer.
Just my opinion, and it works for me.