I saw this in a television advertisement two days ago.

Problem:
“Pair” is a singular noun being used incorrectly as a plural noun.

Explanation:
The TV ad was from EyeMasters, which was promoting the fact that a customer could buy two complete sets of eyeglasses for $99.

A set of eyeglasses — one for each eye — is called a pair of eyeglasses.

Multiple sets of glasses is the same as multiple pairs of eyeglasses.

I believe that the misuse of “pair” as a substitute for “pairs” is consistent with my “Devolution toward Simpler” linguistic hypothesis. It is simpler to say “pair” than to say “pairs”.

For fun, I searched Google for each of the following phrases (with the quotation marks) and got about the indicated numbers of matches:

• “two pairs” — 3,230,000 matches
• “two pair” — 1,080,000 matches

This tells me that Web authors favor “two pairs” over “two pair” by a pitiful ratio of 2.99:1.

Solution:
“2 Complete Pairs for $99”