Anecdotal, but back when I used to use drugs recreationally all of the dealers I used actually placed a very strong emphasis on customer service. Maybe this person has a similar relationship with their supplier. Not all drug deals are conducted from street pushers.
I'm annoyed by the Hollywood depiction implied by this comment. In my version of "back in the day", all the "street pushers" were some of my closest friends, not grumpy thugs.
Correct me if I'm wrong, but you don't usually buy drugs like that literally on the street. It's not unthinkable to have an ongoing relation with a dealer enough to be able to return something that doesn't meet your expectations, if you are an important enough client. Not saying it's common, but I think possible.
If you're dealing with a dealer a bit more upscale than a street pusher, he will have a money back guarantee. It's in his interest since his reputation is everything.