( Girls! ), honestly I need advice on this..?

[QaK]

I like this girl, she works as a Barista at Mcdonald's. I met her first time since couple weeks before, the day I met, asked her if she has a boyfriend - she smiled and didn't answer.

I'm not shy in public. I approached her, talk for a lil bit, she also gave some signs of interest like asking me "Where do you study?" and she kept looking into my eyes, too. But when I asked for her number, she said "Can u have my LINE(app) instead?", then I told her I don't use LINE and I only have Facebook. Then she told me she doesn't use Facebook and asked for my number instead as she told me like she just got a new number, couldn't remember her own number. So, I decided to write down my number on a note, and she said she would call.

After that, one week passed, she didn't call me so I went to Mcdonald's again for several times but didn't meet her at all. By the way, there was a senior waitress who knows the girl I like, and they talked about me. The waitress come to told me that girl "lost my number and she will come again on Saturday".

Due to that Saturday I would be very busy all day, so I went to Mcdonald's very early in the morning that she didn't come yet. I decided to leave a letter(small paper with envelop) to another worker there, to give it to her. The message I wrote inside, was like "since I didn't see you again after the first time, and you told that you lost my number, therefore I gave you my number officially this time, call or text to let me know if you aren't single, because I'm not interested in girls who aren't single." (plus some cocky&funny style in the message).

Now it's four days already and she didn't call (or didn't receive my letter, idk).
I could feel signs of interest while talking to her at first impression, her response is pretty good.
She's the same age as me, 22.
I was calm and good-looking(casual), and i was with a friend of mine at first, ordering cups of coffee.
She's quite-good-looking and looks very feminine (small and cute, not too beautiful)
I'm sure that she's comfortable while talking to me,
maybe she just don't know me and she's not a flirty kind of girl, she may think "too soon for number".

Anyway, this upcoming Saturday I will go to have some coffee at Mcdonald's again as regularly I do as one of my hobbies. I'm always playful and flexible. And i have an appropriate self-confidence.
So, if I met her this time (would be the 2nd time we met each other),
I will talk a bit and at the end I will ask like "Do you remember your own number this time? *smile*"

is it ok? overall, what do you think?
any ideas/recommendations would be so appreciated.. Thanks in advance!
she's not that kind of hot girl, -- i mean i'm better looking than her.. if u know what I mean.

sometimes it's kinda hard to anticipate, even the first impression was so perfect and the instinct told that she's into me.

idk, she may not yet received my letter, otherwise she doesn't want to call me for some reasons..
do you girls think it's possible that she gave me a subtle signal that is like "she wants me to come to meet her again and lemme try to ask for her number again" ?