Copayment rhymes with:
downpayment, nonpayment, payment, prepayment, repayment, arcement, claimant, downpayment, embayment, nonpayment, payment, prepayment, rayment, repayment
Copayment sounds like:
cabinda, cabinet, caffeinate, caffeinated, casavant, cheapened, checkpoint, couponite, covenant, covent