Types of shipping at Japan Post

I live in Japan and ordered something online from the US. I have to return it within 15 days. The box isn't too large and it weighs almost nothing.

On the Japan Post website it says that airmail only takes 3 to 6 days.

I've never shipped with airmail before and I live in the countryside, there's no English speakers here. Is this phrase appropriate to use?

Koukuubin de onegai shimasu.

Will I have to fill out any forms?
If so, will the forms have English?

Have you looked at the JP English wesbite? It has a vast array of info.

Yes, I did. That's where I learned about the air mail. I'm wondering if my Japanese is ok.

Yes, your Japanese phrase is OK.

I don't know if regular airmail gets it to the States in 3 - 6 days; to me it has been more like 7 - 10 days.

If quick delivery back to them really matters, you might want to try EMS (Express Mail Service), for which you can say:
- EMS (ee em es) de onegai shimasu.

Or, you could ask:
- Koukuubin to, EMS de, ikura ka oshiete kudasai. (Please tell me how much it will be with EMS, and via air mail.)

...and after hearing the cost, can pick one or the other :)

Thank you AK!

Thank you AK!