Hello,
Transactions (estimates, invoices, sales orders) deal with Customers. 'Customer' is a unique identifier between Method and Quickbooks. Only the primary and alternate contacts in Quickbooks are sent to Method. Any other Contacts created in Method can be used for CRM purposes only and will not sync to Quickbooks.
You can not create estimates for individual contacts (other than the main contact ) but you can send to individual contacts by modifying the Opt Out Of Billing? option. This option controls which contacts get billing emails such as estimates and invoice notifications. To see exactly which contacts have this option selected you can select to show the Opt Out Of Biling? column, as seen in the image below. If you want a contact to receive billing emails, edit the contact and uncheck this option. If you only want a single contact to receive billing emails, ensure they are the only one with the option Opt Out Of Billing unchecked.
Noel