Enquiry of integrating Google Shopping as Saas feature in website

[Oops Tang]

Hi,

I would like to ask if I would like to integrate Google Shopping to my own Saas website which type of authenticate should I go for. Currently, I have selected Google OAuth to be the authentication but I have a doubt if "Shop owner" wish to offer us the permission to access their merchant account, how can we gain access to their Google Shopping Merchant Account via Content API for shopping.

Besides that, I have come across the calmwebsite API from Content API for Google (https://developers.google.com/shopping-content/reference/rest/v2.1/accounts/claimwebsite?hl=en) and noticed that it requested the API to be filled in with merchant id and account id. May I know how can I retrieve the account id or create an account that has account id? 

Thanks.

Regards,

Oops