When sending invoices from CORE to MYOB AccountRight, you can set the expense account in Integrations > Sync Settings > Accounts. The hierarchy of the accounts in terms of CORE-MYOB AccountRight integration is item-level account first, followed by accounts set at the project-level, then accounts set in the Sync Settings > Accounts screen. If you want all the expenses on the invoices to go to one account, set that expense account in the Integrations settings. The same should be done for the income account. Make sure expense items and activity items don't have any accounts associated. Check CORE Help Center for details.