Thank you for getting back to us.
We did not locate such object in your connection. Please let us know its name.
AccountContactRelation is an object specifically processed by Salesforce. This object stores all related information about accounts and their contacts and vice versa.
In the Account object it is possible to find all related contacts of a certain account, just like in the Contact object you can find all account IDs related to some contact. This data is added automatically by Salesforce.
We are not sure that your custom object will be processed by Salesforce in the way it processes AccountContactRelation.
Also, we would like to mention that you need to add a separate column in both order and billing objects. These columns are to be fullfilled with IDs from your CSV file.
They will be used for lookup when UPSERTing the data into the custom object "order with billing". Standard ID fields are autogenerated by Salesforce and it is required to have special IDs which will be used for looking up by Skyvia.
We are looking forward to hearing from you.