Translation:This shirt is not expensive.
... yeah, but it isn't "tall", just a word that happens to be written the same. Like "letter" (of the alphabet) and "letter" (written message), or desert (dry area) and desert (abandon), and plenty of others. If it helps, think of "expensive" as being "high" in price/costs.
I did, and it was marked as incorrect. I don't know the reason for that, because I thought I didn't have to use a special plural form for that. But it said このくつ for "these shoes." Now I know that shoes is mostly used as a plural, but then how else would I say "these shirts" if not このシャーツ?