We have a customer who mails letters to perspective clients locally. We print and mail the letters directly to customer's perspective clients. She (the customer) believes there should be sales tax charged on these letters.

Charged to HER or charged to YOU?

There is no sales tax on the labor of printing, ( where I am at anyway) there is no sales tax on postage, if you are supplying the paper or postage cards, there is sales tax on that.
** I assume you are billing for all parts of the work separate.

You the seller of the material will owe the sales tax and if you don't bill the buyer, that is more your issue, they buyer while they should be paying it, if they don't, you as the seller would owe the tax on the goods sold anyway.

Charged to her...

Charged to HER or charged to YOU?

Charged to HER..

I agree with Fr Chuck's assessment; no sales tax is due.

Whether sales tax is to be paid on labor is dependent on the tax laws of the particular state and/or municipality where the business is located. According to a Wikipedia article on sales taxes:

"Most states... tax services that are an integral part of producing goods, such as printing or cabinet making.."

Sales taxes in the United States - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Sales_taxes_in_the_United_States)

So john4473: where is the printing business located?