The polymer additive is only an extra that most use for stuff that will be outside. It tends to make the mortar more flexible. You can use plain mortar mix, Quickrete brand is what I use. The Mapai brand is nice to use, but not necessary and don't lose any sleep over it. The Quikrete brand is what brick layers use, and is quite a bit less expensive than the Mapai. When and if you can get the Mapai brand, Slow and I use it and the Mapai grout almost exclusively. I am using the less expensive mortar on my column because I have to use a lot of it.
Hope this helps.

Thank you. That's good to know! I've use Quikrete for other types of projects & it is cheap & easy to find.

I'm too tired from working all day on the fountain to do any Googling research, but if I'm not mistaken, the admixture also adds adhesion. Mapei is very sticky, and just plain thinset w/o the admixture isn't. When I've used thinset w/o the admixture, some things didn't stick, and came off later - on the wall in my workshop. If I were using just plain thinset, I'd mix half water and half admixture for the liquid.

I've ordered things from Lowes Online and chose the option of having my order sent from their warehouse to my local Lowes for in-house pickup and I saved myself from having to pay postage. Maybe that's an option for you?

Addiesue