New memory cards were shipped promptly. However, it appears that USPS lost the package.
I opened up a chat on a Sunday. The MacSales representative was very helpful in taking my information, but (as I expected) there was no one that could help with tracking the shipment on the weekend. However, a MacSales representative contacted me on Monday, as promised.
After exchanging information about the USPS tracking system feedback, MacSales promptly shipped replacement memory cards. This time, by UPS.
Great service and communication all the way around. By the way, the memory cards are exactly as ordered and are working great.