This is because Reverb is a marketplace consisting of many, many sellers. Each Reverb seller accepts either:
Since each payment typically needs to go to a different seller, sometimes multiple payments may need to be made.
If each of the items in your cart is being sold by a seller that accepts Reverb Payments, you should be able to check out just once for all of your items using a credit card.
However, you will need to make separate payments for each item from any of your sellers accept PayPal exclusively.
This applies to purchases made for Reverb LP items, as well.