[guys/girls are welcome] Need advice for this situation, help pls.?

[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 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? honestly what do you think overall?
any ideas/recommendations would be so appreciated.. Thank you in advance! ; ]