SENIOR CITIZEN ID

Good day I have a question po.Ano po ba ang tamang batas for senior citizens when purchasing..I have an issue earlier that my father was trying to buy medicine to one of mercury drugstore and the manager refused to provide the 20% discount because my dad does not have the senior citizen ID.I thought that as long as you have proof such as driver's license ID (that he actually presented)it should be honored.What is the correct law for this?Please help. Thank you.

Republic Act No. 9994.

Implementing rules
5.5 IDENTIFICATION DOCUMENT – refers to any document or proof of being a senior
citizen which may be used for the availment of benefits and privileges under the Act and its
Rules. It shall be any of the following:
a) Senior Citizens’ Identification Card issued by the Office of Senior Citizens Affairs
(OSCA) in the city or municipality where the elderly resides;
b) The Philippine passport of the elderly person or senior citizen concerned; and
c) Other valid documents that establish the senior citizen or elderly person as a citizen of
the Republic and at least sixty (60) years of age, which shall include but not be limited to
the following government-issued identification documents indicating an elderly’s birthdate
or age: driver’s license, voters ID, SSS/GSIS ID, PRC card, postal ID.