As verbs the difference between pay and copay
is that
pay is to give money or other compensation to in exchange for goods or services or
pay can be (nautical|transitive) to cover (the bottom of a vessel, a seam, a spar, etc) with tar or pitch, or a waterproof composition of tallow, resin, etc; to smear while
copay is to jointly pay with another or others.
As nouns the difference between pay and copay
is that
pay is money given in return for work; salary or wages while
copay is a copayment.
As an adjective pay
is operable or accessible on deposit of coins.