Electronic is an adjective.
Electronics is a noun, referring to things like televisions, radios, etc.
A store that sells food is a "food store."
A store that sells electronics is an "electronics store."
An "electronic store" would suggest a store which is electronic, rather than a store which contains things that are electronic.
Think about some other examples:
Pet cemetary or pets cemetary?
Parent-and-toddler session or parents-and-toddlers session?
Towel rack or towels rack?
Cat food or cats food?
Grocery store or groceries store?
The only time the plural form is used is if you have a noun which doesn't have a singular form, as in your first example:
Electronics store (there is no singular form of electronics).
Clothes store (there is no singular form of clothes).
Hope that helps.
Not a teacher.