A letter /e-mail from San Francisco

I have received an e-mail from a mother who lives in San Francisco.

I would say "I recently received..." because it sounds less stiff (excessively formal?), but the way you have it is still perfect English. It sounds a little odd to state sternly that you "have received" an e-mail in one sentence, then use something as casual as an emoticon in a later sentence.

She has been riding her Mamachari (mama bicycle) with her children.

You shouldn't be afraid to use foreign proper nouns. Do remember correct capitalization, though.

I am very glad that she is happily riding the electric Mamachari with her children.

She tells me, "I wish there were a regular importer of electric-assist Mamacharis in the US, because our bike is wildly popular in the city, and many of our friends have asked us where they could buy one.

It might sound more natural to say "She told me..." because you prefaced your writing with a remark about a past letter (thus you would use the past tense). However, if your mother often discusses how she wishes that a company would import mamacharis in the US, then it would be just as good to keep the verb in the present tense. Though, if it is the case that your mother often tells you about the lack of an importer, you might consider saying "She *often* tells me..." to be more clear.

Strangers stop me while we’re riding to ask me where they can get a bike like mine." Hmm, I will be an importer someday.:)

1. See http://dictionary.reference.com/browse/hmm versus http://dictionary.reference.com/browse/hum (be sure to look at the entry referring to the interjection "hum," not the verb) for the important difference between "hmm" and "hum." I will say that in my experience as a native, "hum" is not often used as an interjection, whereas "hmm" is very common. 2. I would say "Hmm, I might have to become an importer someday." instead of "Hmm, I will be an importer someday." because when you say that you *will be* something, that implies a resolution to become something, not a playful/joking possibility.

In her recent e-mail she asked me to find an extra battery for her Mamachari.

The battery is rechargeable, however it is old (maybe 5-6 years old?) and the battery is starting to fail.

The "maybe" shows enough hesitation; you don't need a question mark. Chargeable (http://dictionary.reference.com/browse/chargeable) is a legal term. The word you want is "rechargeable."

I think it is very possible to send a new battery to her because the weight of the battery is under 2kg, and we can use EMS service to ship it from Japan to the US.

I don't know what "EMS service" is, but that could just be me. If you feel that many readers don't know what "EMS service" is, you should elaborate. Please note the added comma after "2kg." Whenever the subject changes after an "and," you will need a comma before the "and." In your sentence the subject changes from "weight" to "we."

Now I am confirming that an NiCD battery is allowed by the US Custom Bureau to be imported.OR: I am now confirming that I am allowed by the US Customs Bureau to send an NiCD battery to my mother from overseas.

If you leave out the "that" a reader might think at first that you are confirming a battery, not a rule about a battery. The sentence feels a little unnatural to me, but with corrections it is perfect English.

Lithium ion batteries are prohibited by the officials to be imported.OR: It is prohibited by the officials to import lithium ion batteries.

This sentence also sounds unnatural. I would avoid using the passive so much (making the subject the recipient of the verb's action) as it is considered poor style.

Anyway, I am glad that my mother informed me so greatly many information about Mamachari in the US.

Her friend, who lives in Seattle, even has an electric Anjelino which allows her to seat two children on it as I always do on my <insert vehicle name here>.

Unless you also have an electric Anjelino, you need to specify what bike you have.

She is living in Seattle.

Saying "Wow, fantastic!" right after this sentence makes you seem as though you are talking about your mother's friend's living in Seattle as being fantastic. If you move this sentence, the meaning becomes clear.

I think it is very possible to send it to her because the weight of the battery is under 2kg and we can use EMS (service) to ship it from Japan to the US.

Strangers stop me while we’re riding to ask me where they can get a bike like mine." Hum, I will be an importer someday.:)

Why would you want to be an importer? Here is a definition of "importer":
a person or business which brings in a product from another country in order to sell it in their country