Permissions of the integration user

Data flow	Permissions
Customers and Contacts	Read on Account and Contact and on the fields Mercura should take over
Requests	Read on the source object (e.g. Opportunity or Case), its fields and its files; edit on the send flag, the link field and the records themselves. If the object is private in your org, the user needs a sharing rule with read/write access, or Modify All.
Status and cancel	Edit on the status field; read on the field holding the cancel values
Quote to Salesforce	Create and edit on Quote (the lines follow the quote); for Opportunity products, edit on Opportunity; read on Product2 and Pricebook2; edit on the fields Mercura writes (e.g. quote status, line description). Plus the system permission "Edit Product Prices", so the Mercura price may differ from the list price. For the PDF on a Case or custom object, read on the record is enough.
New Opportunities	Create on Opportunity; edit on the Account and Price Book fields and on your org's required fields
Orders from Salesforce	Read on Order and its files; edit on the customers' Accounts (Orders follow their Account's sharing) and on the send-flag, link and status fields. Permission to activate Orders is not needed.

Mercura cannot see fields the user may not read. When you save, Mercura checks every configured field and names any missing permission.