Just ordered some, this is still last year's price, they just got back in stock: https://sdbullion.com/10-dollar-face-us-mint-walking-liberty-silver-half-dollars-au-xf
That's a decent price assuming it's all AU/XF. Sold out just a few minutes after you posted, reminds me of the US mint.
Yes, this happened a few months ago too when they showed up in stock again, but I missed out that time. My guess is they have most 90% priced against spot, and the prices vary accordingly, automatically. These specialty 90% might have a hard price entry which they forgot to update. Sign up for the reminder, might happen again!