Is there a difference between a “client” and a “customer”?

I was going through my emails and saw a CVS scam I a get once in a while where the subject line is “Dear CVS Client”. It got me thinking; what's the difference between a “customer” and “client”? Clearly this scammer doesn't know the difference as CVS calls their buyers “customers” yet this scammer says “client”. I worked for Six Flags for a couple years (the pandemic ruined my ladder climbing up the ranks and I lost my job there) and we couldn't call them customers; they were “guests”. So, yeah, what's the difference – if any – between being a client, customer, or other?