Apostophe Police

Authors of all those signs in store windows that say, "Apple's 4/$1" or "Pant's 50% off," can avoid the apostrophe police if they consult this flowchart to determine whether or not an apostrophe is necessary. http://apostrophe.me/ Students, however, will still need guidance from a style manual.

In some style manuals, you do not use an apostrophe for plurals of an abbreviation or a number, which contradicts the first example:
The MLA Handbook for Writers of Research Papers: "Do not use an apostrophe to form the plural of an abbreviation or a number." Examples include acronyms such as PhDs and VCRs.

The Chicago Manual of Style: "So far as it can be done without confusion, single or multiple letters, hyphenated coinages, and numbers used as nouns... form the plural by adding s alone." Examples are SOSs, CODs and IOUs.

As the flowchart says, be consistent.