Found this after some digging
http://pe.usps.com/text/pub52/pub52c4_005.htm
42 Intoxicating Liquors
421 Definition
Intoxicating liquors are drinkable beverages that have 0.5 percent or more alcoholic content by weight and are taxable under Chapter 51 of the Internal Revenue Service (IRS) Code.
http://pe.usps.com/text/pub52/pub52c4_006.htm
422 Mailability
422.1 Nonmailable Matter
422.11 Intoxicating Liquors
Intoxicating liquors having 0.5 percent or more alcoholic content are nonmailable. Taxable liquors (as defined by Chapter 51, Internal Revenue Service Code) with 3.2 percent or less alcohol, including those obtained under a prescription or as a collector’s item, also are nonmailable. The prohibition of the mailing of intoxicating liquors is contained in federal law (18 U.S.C. 1716).
422.12 Promotional Materials
Advertising, promotional, or sales matter that solicits or induces the mailing of intoxicating liquors also is nonmailable. See DMM 601.7.4.1.
422.2 Mailable Liquors
422.21 Products Not Categorized As Intoxicating Liquors
A product containing an intoxicating liquor is mailable if it conforms to the applicable requirements of the IRS and the Food and Drug Administration (FDA), and if it is not a taxable alcoholic beverage, poisonous, or flammable.
The following are examples of products that may be mailable:
a.Cold remedies.
b.Cooking wine.
c.Mouthwash.
422.22 Exempt Mailings Between Federal and State Agencies
Intoxicating liquor is exempt from the prohibition against mailing when it is sent between employees of federal or state agencies who have an official use for the liquor, such as for testing purposes. This exemption is based on the intent of the law to prevent liquor from being transported to prohibited jurisdictions for consumption and to ensure that all proper tax revenues are paid.
So I guess as long as you claim beer to be a cold remedy, you are OK.