Rein here from Shopify. Can I ask what payment details specifically require your Merchant ID? If it is Shopify payments you do not need to provide it.
That being said, if you are looking for it, you would need to reach on out to our support team through our help centre here. This will allow us to authenticate your details in order to provide you with your Merchant ID.
In terms of your Secret Key, I am not sure what this pertains to, it is likely something we do not have access to, however feel free to either specify here or let our support team know when you are chatting with them and they can guide you to finding it.