Wow. So a customer calls to inquire about pricing, stating that the prices charged didn't align with those on the website and the promised 20% discount. She immediately gets an email saying that the difference would be credited. Then a day goes by and there's a voicemail stating that she was in the wrong. (No thank you for the order, nor inquiry about how the wines were tasting.) The last time I felt like this about Santa was circa 1979 when I had a healthy skepticism about not-so-jolly red men in suits with fake beards.
I really don't understand how I got a better deal than the promised 20% case offer when the $20.00 Chardonnay is marked down to $18.00 on the winery's website and I was charged $19.20 per bottle. Even using the original higher $20 rate, this is a 4% discount -- not 20%. (I won't bother going into details of how the other 20% discounts don't add up.)
When I receive an email offer marketing wine, a winery should know that a normal next step in the customer buying process is to check prices online. The prices listed online are in effect the promise made to the customer.
If I was incorrect, why not politely explain my error and take the opportunity to ask about the wines I've just purchased? This would change the tone of the brand conversation -- it is quite possible I'd want to buy more or sign up for a wine club. This is actually an opportunity to create loyalty here by developing the relationship. Versus assume the customer calling with a question wants to pick a fight. (More on this in a moment.)
From a customer service lens, the reason behind a service error doesn't matter. A business might choose to briefly explain why the error occurred and what has been done to ensure it won't happen again. The main focus should be on correcting the problem -- this is both the right thing to do given the brand promise, and provides a good return on investment when done well. The service recovery paradox shows that when a customer's negative experience is solved successfully, her loyalty increases.
In Harvard Business Review's "The Profitable Art of Service Recovery", the trio of authors demonstrate how a Club Med manager turned a group's horrendous travel experience into "the most fun they'd had since college." He went much, much farther than would be expected, especially since the airline delays were not under the resort's control. The vacation that should have been ruined before it started became a story of delight that has no doubt been told many, many times over.

Did I return Santa's call? No, because it's not worth it to me to argue over the money. Especially when I'm dealing with an interaction that should be delightful. It is worth it to me to take my wine business elsewhere -- after all, we're all in the hospitality business first; the epicurean industry second.