Q: Does Apple not sell these memory upgrade cards?

That is because if you want to pay for Apple memory you need to take it in to your local Apple Store. You can make an appointment at the Apple Retail Store - Genius Bar and take it in for service. You can also look for an Apple Store - Find a Store - Apple.

However, if you wish to save some money you can install memory yourself. Any of the three following memory suppliers are reliable vendors and are able to identify the correct memory needed for your model: OWC, DataMem, and Crucial. If your are in the USA, then I would recommend OWC.

I also would add that if not purchasing from OWC, make very sure you purchase

Mac qualified RAM.  Generic PC type RAM does not always work well.

A word of advice, stay away from PNY RAM as it is trash, even though it says

it is Mac compatible!!!!!!!!!!!!!!!!!!