My opinion about BTC

I really doubt anyone here knows the actual reason of charge backs and how to prevent them.

As in any purchase, you should have a receipt, an an invoice. if you do this, and submit them to Paypal when the transaction is done, you're safe.
They basically make sure that Paypal knows that the customer paid and received his item.
Unless you do this, you have no proof he received anything, and the best way is to use WHCMS, even if it's not a store, have it on a local host.

My opinion.