I have an update, if anybody’s interested.
The issue was with VISA, not with CVS. Apparently lots of VISA debit card users were affected by it, at several different merchants. Each victim was charged exactly $23,148,855,308,184,500.00.
The folks at VISA have removed the 23-Grillion dollar charge, but not the $20 negative-balance fee. They promise to do so “as soon as this is all sorted out.”