Pay For You/Buy From You

[Bobbert]

I don’t think I was being understood when I tried to pay for some things for an older person. I would say my first statement, and then have to repeat it another way to make sure I was understood; but I don’t think I was saying it correctly, and I was probably just adding to the confusion when I tried to say it another way.

Are any of the sentences below correct and/or understandable? If not, what is the best way to say them using the same original verb?

I used the Tú familiarity because that’s the first thing that came out of my mouth, but I probably should have spoken to the person in Usted. So please show me the correct Tú as well as the Usted forms.

How close did I get? This is what I said:

I'll pay for your medicine for you.
Te pagaré la medicina por ti.

I'll pay for it for you (with an emphasis on “for you.” In other words, don’t open your purse and try to pay; I want to pay on your behalf.)
Te la pagaré por ti.

I'll buy your cell phone from you in a few days.
Compraré tu celular de ti en unos pocos días

Don't worry. I'll buy it from you.
No te procupes. Lo compraré de ti.

Input and explanations and appreciated.