There are a couple of points to note when dealing with Person Accounts:

• There will NOT be a Sync Contact button on the Person account record – this WILL NOT affect the ability to sync your Salesforce Opportunity -> Saasu Sales Order/Invoice
• The sync of a Person Account will ONLY occur when the Person Account is specified as the Primary Contact Role on an Opportunity.
• The Contact address fields will be synced to Saasu, not the Account address fields. For more info, check here.