Mega Mart put through the charge in Jamaican dollars and my bank converted to CDN. The rate is very close to the rate I paid at the ATM & to buy the Jamaican dollars before I left. No extra charges, sure the conversion is buried in the rate, but that is also the case when I withdraw cash from the ATM. These are the rates I paid, not a big difference at the end of the day:

JA$ bought in Canada .011700
Scotia ATM wd in Negril .011002 & .011038
Visa charge (Mega Mart) .011037