They still don't understand me

I noticed that my glasses case is beginning to get worn around the edges. It’ll last a while, but since I had a hard time finding such a small one I figured I’d better start looking when it’s convenient. With that in mind, I stopped into a little shop near home and said, in Russian, “I came to see if you have a case the size of these glasses.” I took them off and showed them to her.

“What diopter?” she asked me.

“I just want a case,” I replied.

She walked over and opened up a display cabinet. I stepped behind her and saw that the cabinet contained glasses that looked vaguely similar to mine, but no glasses cases at all.

“No,” I corrected her, “I just want a case.”

“A case!?” she asked incredulously. “They’re over here.” (She only had big ones.)

I walked out of the store trying to figure out the difference between what I said and what she said. I played it over and over in my mind. They sounded about the same to me, and I couldn’t immediately identify the important difference. Finally, I figured it out: I said “fotliar” and she said “fotlyar.” They probably look about the same to my readers who don’t speak Russian, and they sound about the same to me. But I got tripped up, again, with a diphthong. I’m not sure diphthongs even exist in Russian, and the second variant, the Russian spelling, includes a vowel we don’t have at all in English: “ya.” Yes, “ya” is a vowel, and I’m told it’s not a diphthong. Here it is: “Я.”

I’ll probably get directly to the glasses cases next time I try.

[Addendum, a few days later: Alla points out that my REAL mistake was in the first vowel. It's futlyar, not fotlyar.]